someone put the shit on my head, what i need to do?

I am woking in a small company. My job was replaced by a man two months ago since I need to take two weeks' vocation. When I came back,this guy left but did not return the company's key. The female boss mentioned this several times to me. Yesterday the  boss came to me seriously and said the guy said he left the key in my desk when he left. I told the boss I didn't see any key in my desk. I feel so uncomfortable, what I can do for this?