Try to change the length of the snooze for the alarm, my wife's phone had the same problem. When I opened the snooze menu it had a value called "choose the snooze-time" selected. After changing it to 5min the alarm clock started working again. The strange thing was that the option "choose the snooze-time disappeared from the lost.
Some people used Status Menu Extension for N9 (http://talk.maemo.org/showthread.php?t=85816) and had infinite alarm loop, and now some people have no alarm at all? Gosh, I think we tweaked and hacked our phones a bit too much, people xD
Try restoring default settings. Sometimes this is known to work for stuff like these.
