This has been asked before. Here is the answer that was given.
Go to the folder in which your catalog file (with the extension *.lrcat) and its accompanying Previews folder (whose name ends with .lrdata) By default they are in C:/..../My Pictures/Lightroom, unless you have changed that. Move the lrdata folder to your desktop. Launch LR and it will create a new lrdata folder. To save LR the task of rebuilding all the previews, close LR and copy/paste the folder from the desktop on top of the new lrdata folder. Generally this is enough to solve the problem, but if it doesn't, let LR build new previews.