Problems with Time Machine

My daughter dropped down my external hard disk which I used as back up. I reconnected it to my Mac. Files appear to be there. Nevertheless, Time Machine no longer works. When I launch it, the "Restore" button does not activate and the Mac blocks. Also, when launching back up, it is really slow. I left back up running overnight and it onlyget to "preparing 624 items" (out of more than half a million). Help please ! (I am a Mac illiterate, be patient please !)