First, be sure that Auto Remove Orphans (Opt|Pref|Thumbnails) is off.
If other programs (eg Explorer) can access that drive, ThumbsPlus shouldn't have any trouble with it either. Are you sure that you did not also change the server name (of the NAS) or the volume label (of the drive in the NAS)? Either of those could cause TP to no longer recognize it, too. When you lookup the old entry for the NAS drive in your tree panel, you can right-click on it and select Volume Properties to see by what label that drive used to be known (if you want to restore that name, be sure to exit TP and do it using Explorer or some other tool. Then when you launch TP and it recognizes the drive again, you can use TP to change that label back to the desired new name, and at that point TP can update its own records so that it can continue to recognize the drive and its contents in the future).
See also Additional References > Database volume matching in the helpfile.
If this doesn't help, please let me know what kind of password you changed, and which application or part of Windows is asking for that password.
