All previews used by Lightroom can be deleted. They will be rebuilt as needed.
In Develop the Camera Raw cache is used for previews. In the Lr Preferences you have an option to "Purge Cache", and you can also move it to a different location.
In Library the previews are kept in a separate folder that sits beside the catalog. This can simply be deleted.
If that doesn't do it, reset Lr preferences by deleting, moving or renaming the prefs file (see here: http://helpx.adobe.com/lightroom/kb/preference-file-locations-lightroo m-4.html )
