US Post office

I am the holder the holder of a US post office box rental
and while I was away on vacation for approximately 3 wks
someone came to the post office claiming that they had lost their key, and the clerk at thew post office gave a new key,and my mail was stolen.
the Post office supv. said that it was a new clerk and did not know better. the lock was changed as soon as I learned of the occurance.
Do I have a case against the Post Office?

What premise do you propose to take action on?
Theft? Nope.
Negligence? Maybe, but only if you have a LOT of money to waste.
Stupidity? See Negligence.

And, unless the mail that MIGHT have been stolen included irreplacable cash, artwork, gold, bearer bonds, etc. what are your 'damages'??