1998 Lexus GS 400 will not start

I'm almost positive I need a major tune up. But my car has a relay switch that stayd on after the car is off. So it drains the battery. I have to unhook the battery if not dribing for longer than about 4 hours. The battery wss fully chsrged car was running just fine and sll of a sudden it wouldn't start. It turns over but it will not cut on. Help me please!