I used to work for a pet store, and I have 3 mature leopard geckos now. The tail coming off will not harm the gecko; sometimes they fall off due to stress or simply get it caught. Some breeds of geckos will grow their tail back, some will not. Leopard geckos will. You cannot put the tail back on; if you leave it in the cage chances are the gecko will eat it (ya gross I know). You should watch for several days and see if his coloring changes. Stressed out geckos will stay a dark color, almost black. If he is seriously stressed out then you need to make sure his temp is ok, he has food but not too much, and there is proper lighting in the cage for that species (some are nocturnal and will freak out if the light is being left on constantly). If his color does not return to normal then I would seek a reptile vet. Good Luck and feel free to write me if you need more info!
at 3:26 PM on Jun. 22, 2009