What do you mean by stopped working? (Stupid question, I know, but you see what I'm getting at.) If it just won't retrieve your mail from a particular address it could be because the mail server is down (or has been changed) or they've withdrawn POP3 access.
Sometimes it refuses to appear if one session closed irregularly and you've since begun another. Go into Task Manager and look for all instances of msimn.exe in the process tab and click "End Process" for each until it either appears or there are none running (at which point you should be able to start a new one, bearing in mind that any work in progress will naturally be lost.)
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