If it cranks but does not start, and seems to do it only after the car has been sitting for awhile, it could be a fuel pump or a pressure regulator (on some models this is integrated into the fuel pump). You might try cycling the key - without starting it once or twice before actually trying to start it. If that eliminates the problem, than it is definitely fuel pressure related.
It could also be a weak battery or if it does not crank at all than it may be in the ignition switch.
You will likely need to have it checked out by a certified technician.
〉 Answered on May 18th, 2009 by Bogi Lateiner, Owner and Technician at 180 Degrees Automotive