A Hyundai's "check engine" light comes on when the onboard diagnostic system detects a problem that can effect the way the engine runs. The "check engine" light has a code associated with it explaining why the light came on. Mechanics and car dealerships have scanners to read these codes and tell you what is wrong.
There are many possible reasons for the "check engine" light coming on. The gas cap could be damaged or just not tightened correctly. Water could have gotten into the engine. The spark plugs could be worn out or the catalytic converter may need to be replaced. It can often be caused by a problem with the ignition or the braking system. The corresponding issue doesn't necessarily need to be fixed right away. Certain issues, such as the oxygen sensor malfunctioning, impact performance but the car is still drivable. The car should be taken into a mechanic or dealership as soon as possible to make sure there isn't a serious problem. The code can usually be determined with a scanner within an hour. Certain dealerships check the code free of charge. The light turns off automatically if the problem stops occurring, although it doesn't happen instantly. It can take a couple hours of drive time for the light to turn off.