Edit menu, General Preference: Launch to factory state does not fix the problem?  Can you view your UI dots and have you saved any configuration wherein the library palette is visible?

Using Windows 7 there is a folder at: C:UsersUSER NAMEAppDataRoamingPoser 7

The folder is hidden but if you type into the Exlorer address bar you can open the thing and see it's contents.  The preferredState.pz3 file is there.  Might be if you cut and paste those files into another folder and try booting Poser the program will revert back the original state.  I'd move them rather than delete them in case something goes wrong.

Once you get things back again you might save that original configuration in one of your UI dots.

Best Wishes,
Rªnce