Is the ballast a digital or magnetic one?
If magnetic? I would unplug it and open the box and check the capacitor.. and all componets for scortch or burning,even melting. That would be where the problems start. Even at the plug end. Is it a 3 prong plug? Or has it been broken off,GROUND PRONG.. Is it HPS only? Is it under warranty? New? Used?

If used,then it has probably been used past it's lifespan.. Componets should be replaced,then. Depending on how long it was in use.