If it occurs again and if you are running Windows XP, press
CTRL-ALT-DEL and check the processes tab for the process:
msmsgs.exe. If you have that process then that means that
Windows Messenger is running and is started automaticly after MSN Messenger. This can cause the exact error/beheviour that you experienced...
To prevent Windows Messenger from starting up automaticly see:
CookieRevised's reply to I am about to rip my hair out over MS Messanger