Can cats get amnesia?

He won't eat, drink, use the litter box, and he keeps panting. We took him to the vet and they don't see anything wrong with him. He's not even two years old yet. My sister has cerebral palsy and isn't very smart sometimes, and she smashed him with a clothes hamper, I didn't see him the whole day and I became worried. I found him behind some boards in the laundry room and put him on the bed. He wouldn't move or act like he used to. The next day he just started hissing at us and biting at us. He hid under the love seat in our living room and he never came out from under there. Then we took him to the vet and they are keeping him over night. He used to be the sweetest kitty he would always just rub against everyone and loved being the center of attention and after my sister did that he became anti social and just started scaring us by hissing and biting. He's a Scottish fold cat. This was the first time he's ever hissed at anyone or bit someone in a non playing way.