First of all, your original post said "Most of the songs in my playlists disappeared"--does that mean when you view the playlists within MusicBee, the files are marked as missing?
As far as the Notepad files thing you can associate anything with Notepad but that doesn't mean it's a text file. In the example you gave at least, it sounds like what frankz said is what happened--you opened a binary *.mbp playlist file into Notepad. If you were doing that before and attempting to re-save or alter any of the contents, they become wrecked.
If it's been so long since you even used MusicBee, I would recommend a complete uninstall, a re-install of the latest PORTABLE version from this website to a folder like C:\MusicBee, and then recreate your library and playlists. Don't even try to carry over any old settings, etc. Fresh is good, even if it takes an hour or so.