What happens if a private pilot's license isn't used for some time?
In the US (under the FAA) there are 2 things that must be maintained (lets assume this is a VFR license). 

First there are some basic flight requirements. You don't need an instructor to complete these. It basically boils down to 3 take offs and landings every 90 days to be current to cary passengers. Here is the full legislation. 


  Sec. 61.57
  
  Recent flight experience: Pilot in command.
  
  (a) General experience.
  
  (1) Except as provided in paragraph (e) of this section, no person may
  act as a pilot in command of an aircraft carrying passengers or of an
  aircraft certificated for more than one pilot flight crewmember unless
  that person has made at least three takeoffs and three landings within
  the preceding 90 days, and--
  
  (i) The person acted as the sole manipulator of the flight controls;
  and (ii) The required takeoffs and landings were performed in an
  aircraft of the same category, class, and type (if a type rating is
  required), and, if the aircraft to be flown is an airplane with a
  tailwheel, the takeoffs and landings must have been made to a full
  stop in an airplane with a tailwheel.
  
  (2) For the purpose of meeting the requirements of paragraph (a)(1) of
  this section, a person may act as a pilot in command of an aircraft
  under day VFR or day IFR, provided no persons or property are carried
  on board the aircraft, other than those necessary for the conduct of
  the flight. (3) The takeoffs and landings required by paragraph (a)(1)
  of this section may be accomplished in a flight simulator or flight
  training device that is--
  
  (i) Approved by the Administrator for landings; and (ii) Used in
  accordance with an approved course conducted by a training center
  certificated under part 142 of this chapter.
  
  (b) Night takeoff and landing experience.
  
  (1) Except as provided in paragraph (e) of this section, no person may
  act as pilot in command of an aircraft carrying passengers during the
  period beginning 1 hour after sunset and ending 1 hour before sunrise,
  unless within the preceding 90 days that person has made at least
  three takeoffs and three landings to a full stop during the period
  beginning 1 hour after sunset and ending 1 hour before sunrise, and-
  
  (i) That person acted as sole manipulator of the flight controls; and
  (ii) The required takeoffs and landings were performed in an aircraft
  of the same category, class, and type (if a type rating is required).
  
  (2) The takeoffs and landings required by paragraph (b)(1) of this
  section may be accomplished in a flight simulator that is--
  
  (i) Approved by the Administrator for takeoffs and landings, if the
  visual system is adjusted to represent the period described in
  paragraph (b)(1) of this section; and (ii) Used in accordance with an
  approved course conducted by a training center certificated under part
  142 of this chapter.


Every 2 years you will also need a flight review which is administered by an instructor. 


  Sec. 61.56
  
  Flight review.
  
  (a) Except as provided in paragraphs (b) and (f) of this section, a
  flight review consists of a minimum of 1 hour of flight training and 1
  hour of ground training. The review must include: 
  
  (1) A review of the
  current general operating and flight rules of part 91 of this chapter;
  and 
  
  (2) A review of those maneuvers and procedures that, at the
  discretion of the person giving the review, are necessary for the
  pilot to demonstrate the safe exercise of the privileges of the pilot
  certificate. (b) Glider pilots may substitute a minimum of three
  instructional flights in a glider, each of which includes a flight to
  traffic pattern altitude, in lieu of the 1 hour of flight training
  required in paragraph (a) of this section. (c) Except as provided in
  paragraphs (d), (e), and (g) of this section, no person may act as
  pilot in command of an aircraft unless, since the beginning of the
  24th calendar month before the month in which that pilot acts as pilot
  in command, that person has-- 
  
  (1) Accomplished a flight review given
  in an aircraft for which that pilot is rated by an authorized
  instructor; and 
  (2) A logbook endorsed from an authorized instructor
  who gave the review certifying that the person has satisfactorily
  completed the review. (d) A person who has, within the period
  specified in paragraph (c) of this section, passed a pilot proficiency
  check conducted by an examiner, an approved pilot check airman, or a
  U.S. Armed Force, for a pilot certificate, rating, or operating
  privilege need not accomplish the flight review required by this
  section. (e) A person who has, within the period specified in
  paragraph (c) of this section, satisfactorily accomplished one or more
  phases of an FAA-sponsored pilot proficiency award program need not
  accomplish the flight review required by this section. (f) A person
  who holds a current flight instructor certificate who has, within the
  period specified in paragraph (c) of this section, satisfactorily
  completed a renewal of a flight instructor certificate under the
  provisions in Sec. 61.197 need not accomplish the 1 hour of ground
  training specified in paragraph (a) of this section. (g) A student
  pilot need not accomplish the flight review required by this section
  provided the student pilot is undergoing training for a certificate
  and has a current solo flight endorsement as required under Sec. 61.87
  of this part. (h) The requirements of this section may be accomplished
  in combination with the requirements of Sec. 61.57 and other
  applicable recent experience requirements at the discretion of the
  authorized instructor conducting the flight review. (i) A flight
  simulator or flight training device may be used to meet the flight
  review requirements of this section subject to the following
  conditions: 
  
  (1) The flight simulator or flight training device must be
  used in accordance with an approved course conducted by a training
  center certificated under part 142 of this chapter. 
  
  (2) Unless the
  flight review is undertaken in a flight simulator that is approved for
  landings, the applicant must meet the takeoff and landing requirements
  of Sec. 61.57(a) or Sec. 61.57(b) of this part. 
  
  (3) The flight
  simulator or flight training device used must represent an aircraft or
  set of aircraft for which the pilot is rated.


The license its self (as the laws currently stand) is good for life, barring you having it revoked for some reason you will not need to fly your check ride again (unless you go for something like an instrument rating or a higher license). 

Until recently here in the US you also needed a valid medical certificate. However recent regulation change has altered that a bit. You can read up on some of the changes here as they are still in progress. 

On a bit of the more practical side there are some things to consider. For example many people here in the US rent aircraft. Your local flight school or rental facility may impose other rules on flight currency for you to be allowed to rent a plane. Likewise if you own a plane your insurance company may require certain experience or other things in order for you to be allowed to fly your plane. Generally this is based on your experience as well as time in type.