why you can't open it ? you tried double clicking on the icon ? I suppose you also enabled the 'start in the tray' option which makes amsn start minimized in the tray icon...
if you don't know how it works and don't want to play with it, the easiest way to fix this is by removing your configuration files, just move the directory ~/.amsn (delete or move, but move is better so you can keep a backup of your log files so far) by doing in a console something like this :
mv ~/.amsn ~/amsn_backup