You're off topic for this forum, which is dedicated to Windows Live OneCare.
However, I can answer your question.
When you click to mark a message as unsafe, it is permanently deleted and cannot be recovered.
However, it also adds the sender to you blocked senders list. Go to options, then Safe and Blocked Senders, then Blocked Senders. Scroll the list to find an address that looks like it might be the sender you accidentally blocked. Make a note of the address and then highlight it and click Remove from List.
-steve