if they are injured they can stay closed up for a day or 2
if you dont see any movement within 48 hours
remove him from the tank and smell him , you'll soon tell that he is dead or alive
the only thing you can do is leave him alone for the 2 days and hope hes ok , mine has a large cracked shell with nearlly all her body exposed after a few chunks out of her after falling out the tank and getting found by my dog , this happened 8 months ago and she has layed eggs since then and still seems happy , her shell is slowly healing over where it is missing and she has some dents in her skin but she seems ok and still moves around quite speedily in the tank like she owns it , budging all the others out the way for the food