I just saw a plane drop off online radar; should I do anything about it?
Really there's nothing you should do in cases like these.

Flightradar24, FlightAware, and similar services should not be used for flight safety purposes, and most of them specifically state so in their terms of service (such as sections 12 and 14 of Flightradar24's terms and conditions). Their sources may go down for whatever reason -- that does not mean that the plane has crashed.  

These services can only track what ATC controllers can (for the most part), so it's more likely that if there is a real issue, that they'd notice without the general public making a fuss.



In this specific case the flight was being tracked using MLAT (designated by the radar type T-MLAT, which is shown in your animated image file). MLAT flight track accuracy relies on the accuracy and number of the receivers in the area, and according to Flightradar24's website, MLAT generally only works above 10000 feet:


  MLAT
  
  In some regions with coverage from several FR24-receivers we also
  calculate positions of aircraft with the help of Multilateration
  (MLAT), by using a method known as Time Difference of Arrival (TDOA).
  By measuring the difference in time to receive the signal from
  aircraft with an older ModeS-transponder, it's possible to calculate
  the position of these aircraft. Four FR24-receivers or more, receiving
  signals from the same aircraft, are needed to make MLAT work. That
  means that MLAT coverage can only be achieved above about 10000-20000
  feet as the probability that signal can be received by four or more
  receivers increases with increased altitude.
  
  MLAT coverage is today limited to some parts of Europe and North
  America, but expanding fast.


So as the aircraft descends below 10,000 feet there is an increasing chance that you will lose it on Flightradar24's tracking system.