The Engine Computer detected a problem, set a diagnostic fault code, and turned the Check Engine light on to tell you. Many auto parts stores will read the codes for you for free. Those will indicate the circuit or system that needs further diagnosis.
There are well over 1000 potential fault codes so there's no way to guess what it is. You can make some generalizations by the way the Check Engine light acts. About half of the codes won't even turn the light on. The light must turn on when the problem detected could adversely affect emissions. If the light turns off while you're driving, the problem went away for now and those are the least serious. If the problem stops acting up while you're driving but it was a little more serious, the light will stay "latched" on until you stop and restart the engine, like you described. If it's still more serious the light will be on any time you run the engine even if the problem hasn't acted up in a while. For really serious problems the light will flash while you're driving. That's when you must stop the engine right away to prevent overheating the catalytic converter and damaging it. That is caused by too much raw fuel going into the exhaust system.
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Saturday, July 27th, 2013 AT 6:05 PM