For the love of tune up
The computer will first try to correct the problem itself, but when it can't, it lets you know by illuminating the check engine light. Pull over, turn off your car and check your gas cap before doing anything else. (To be safe, you should also check for low oil pressure or overheating before restarting your car. You should not operate your car under either of those circumstances. ) If that does not reveal the problem, then your wisest move is to take your car to a repair shop as quickly as you can, even if your car still appears to be running smoothly.