What password? What version of Messenger Plus!? Please be specific if you ask some help/support.
Do you mean the password used for encrypting your chat logs? In that case there is no way to retrieve it.
Do you mean the password used to lock the Plus! preferences? Then see:
CookieRevised's reply to Preferences Password
Do you mean the password to unlock Messenger again after you've locked it? Then you can kill Messenger via Task Manager and restart it again:
* Press Atl+Ctrl+del to open Task Manager
* On the Processes tab right click "msnmsgr.exe" and choose Kill Process
* Close Task Manager
* Now you can run Messenger again
(also remember to change or remove the password in the Plus! Preferences for Messenger locking)
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