First, be sure the Pose folder is really there, by looking for it outside of Poser.

The folder could have been deleted or renamed by accident.

Then, if you're in Poser 6 or 7, find and delete the file 'libraryPrefs.xml'.
Sometimes this file gets messed up and Poser doesn't know where
to find things.

Then restart Poser.   It will build a new default library setup; you'll have
to add back your external runtimes if any. 

My python page
My ShareCG freebies