With LR closed move the preferences file to the Trash as described at the below link. You will lose your Preferences settings, so you may want to record them if unsure what you have changed from the default settings.
http://helpx.adobe.com/lightroom/kb/error-changing-modules-lightroom
DO NOT RESTART LR!
Next uninstall LR and then completely shutdown your system and turn the power OFF. Your catlog(s) will not be removed, so no worries.
Restart your system and reinstall LR using the latest version download available (LR5.4, LR4.4).
What version of LR and OS X are you using?