If your panel happens to be a Federal Pacific..
This is a typical problem. The plug in breakers are prone to burning off at the contact point and the breaker gets loose.
The best fix is to replace the panel, but second best is to get a new breaker and find another slot to install it in.
[URL="http://www.houserepairtalk.com/announcement.php?f=39&a=6"][size=3]The Ten Commandments of House Repair Talk[/size][/URL]
[URL="http://home.bellsouth.net/p/s/community.dll?ep=16&groupid=289528&ck="]Square Eye's home page[/URL]