What if a commercial airline pilot misses their flight?
Airlines try to maintain some ratio of crews to fleet to provide surplus bodies to cope with this sort of thing.  For Regionals, where the average aircraft hours are around 2500 hrs per year, and each pilot flies somewhere around 800-1000 hrs per year, you need 3 crews per a/c just to meet the baseline operational need.  Add absentee time for vacation requirements and booking off sick, training etc, and you need 4-5 crews per a/c to be able to sure that you can keep each airplane running 2500 hrs/year.

As a result, Regional operators will try to keep about 8-10 pilots on staff per aircraft and if they are below that, problems start to happen.  You can get by somewhat with crews working overtime, but there are limits to that due to regulatory duty limits.  Airlines have a reserve system to organize crews to make sure there are available idle bodies to step in if required.  

There is "reserve" and "ready reserve" duty.  Reserve duty is being on-call at or near home and able to report within a reasonable time of being called, like 2 hours.  These are to cover book-offs with advance notice, like a pilot calling in sick 6 hours before.  Ready reserve duty is where the pilot has to go to the airport crew ops office in uniform and basically hang around ready to go on short notice for someone who doesn't show or who calls in at the last minute. 

So:


Yes probably, if there isn't a ready reserve pilot on hand.  
They'll stick in a guy/gal on ready reserve if they have one.  If not, the flight will get delayed or cancelled.
See above.
Booking off sick or calling in because you had a traffic accident or home crisis isn't a problem.  The problem is failing to call in and leaving people hanging.  You better have a very good reason for not showing up and not calling in.  That would result in some kind of disciplinary ding on your record, getting worse if you do it more than once.  It would depend on an airline's union contract, or if non-union, the management's disciplinary policy.
In a union shop, for a one time thing, probably not if not repeated.  Union shops give you the luxury of having to screw something up several times to really get hammered for things that are not safety related. It'll depend on contract language; but you could get suspended or let go eventually.  In a non-union shop, it could hurt you in terms of promotion, or if you did it more than once you might get let go more readily than at a unionized airline.  


Bottom line is, the next worst thing to unsafe pilots are unreliable pilots.

In the Regional airline business especially, the shortage of crews is really starting to bite and airlines are not able to maintain the necessary staffing "fat" as the flow from the bottom thins out and the majors suck up the experienced crews.  Many cancelled flights with Regional operators are due to lack of crews, not mechanical or weather issues.  The 1500 hr "Colgan Rule" requirement in the US has made this that much more severe, but up here in Canada where Regionals can still take 250 hour flight school graduates under special programs where they are groomed in-house, there are still not enough to cope with retirements and flights are being cancelled for lack of crews.

This is all tied to the final stages of the movement of the "baby boom" demographic wave, which is now resulting in a flood of retirements of airline capts born in the mid 50s hitting retirement age, which should finally start to taper off in the next 10 years once the peak of the wave is mowing the lawn at home.  Everybody's vacuuming up pilots from the levels below them, resulting in shortages all the way down the chain.