Why won't my deadbolt unlock and how can I fix it?
You'll have to remove the dead bolt and thumb turn mechanism in order to inspect it. It could just be gummed up in which case it needs to be cleaned and lubricated, but also common is someone forcing the lock and bending a component.  If bent, you can try and straighten whatever is bent with a pair of pliers, but if it bent that easily it is probably a low-end deadbolt in which case it's probably better to replace it.  A locksmith can key a new deadbolt to your current key if that is a concern.  There might also be debris lodged in the mechanism which can be removed.