Omgg, someone please help me!! Computer Trouble :O?

so this morning i left for school, and i shut my Mac... when i came home 4 hours later, i opened it and now the screewn is gray almost looked like some put the brightness all the way down, but no thats not what happened! has this happened to anyone?? if sooo someone please give me advice on to what i should do cuz i was just on the phone with apple and he said he couldnt help me. basically it has full charge but something happened and i now i cant use it!!!!