When I'm driving my car light comes on and says key not in car now the car won't

Hello, Pop the key open and replace the battery. If I remember correctly, the battery is a CR2032, and the key may take 1 or 2 of them. Hyundai does have a re-learn procedure, however, this is just a quick fix that will not last since the system should not have forgotten the key to begin with. You can look in the owners manual, find the location to insert the key fob into the vehicle, and start the vehicle normally. This will allow the vehicle to recognize the key. If replacing the batteries, inserting the key fob, or using the other key does not help, your best bet is to contact the dealer and explain your situation. They may have a software patch available, or they may need to diagnose the issue. Just FYI, the antenna that acknowledges the key is present has problems at times. You can try to touch the key against the start button to start the vehicle as well, but I think that only works on 2013 and newer Hyundai models.