Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
When the Check Engine light comes on, this is the on-board computer's way of telling you that it sees a problem in one of the monitored systems. Unfortunately, there are literally 100's of parts/sensors monitored and to take a "guess" just isn't very efficient. A very common problem is, in fact, a losse, missing or faulty fuel cap. If you are sure the fuel cap is good, and is on tightly, the light SHOULD reset itself within 50-75 miles of normal driving. If the light has not reset itself, you next best bet is to have the "p-codes" read from the on-board computer. These p-codes are used as a "starting point" to begin the diagnostic process and find where the actual problem lies. You can have the -codes read for Free at any local "big chain" parts store (like AutoZone). Once you have these p-codes you can more accurately and efficiently diagnose the problem, and begine the repair procedure.