In case you have trouble viewing your Papers library correctly, for example, if the left-hand side library panel is missing, try removing the Papers2 preference file Library/Preferences/com.mekentosj.papers2.plist from under your home directory. Note that when you then open Papers2 again, you might have to connect it to the location of your library the next time you start it up.

Note that if you use OS 10.7 Lion or above, to access your Library folder, you will need in the Finder to use the Go menu while pressing the option (alt) key, which will add a 'Library' item to the list.