Symptoms: Under some conditions a hidden inspector does no longer become visible when choosing "Show Inspector" from the View menu. Usually the inspector is completely stuck at the right edge of the window and is visible as a double pixel gray line.
Cause: Due to a bug under rare circumstances the layout of the inspector is stored incorrectly
Solution: If you place your mouse over the double pixel gray line at the right edge of the window you will notice you can drag the splitview back to the left and the inspector should show.
If the above instructions do not work for you will need to reset the Papers2 preferences.
Make sure you quit Papers first and then throw away the com.mekentosj.papers2.plist file that you find in your home directory in library -> preferences.
If you have stored your library in a non-default location (i.e. not in home/library/application support) you might have to point Papers again to it by going to the library tab in the Papers preferences afterwards.
Note that if you use OS 10.7 Lion, to access your Library folder, you will need in the Finder to use the Go menu while pressing the option key, which will add a 'Library' item to the list.