Delete the Preview Cache folder. It is found, located, in the same folder as the LR catalog file.
It will be named the same as the catalog file, LighroomCatalogName Previews.lrdata. That is a small L not an i.
Delete the whole folder and look for a file that ends with a .(dot)LOCK extension. If you find one of those delete that also.
Thank you for the response. It appears as if Lightroom 5 is corrupt and may need to be reinstalled. I seem to recall that Adobe wants some kind of notification in order to remove and reinstall their software. Can you tell me anything about removal and reinstallation? I have the Adobe DVD with Lightroom, and my serial numbers...
Not sure on a Mac. But usually you just delete the program folder/file and reinstall.
I suggest you download the most current version of LR 5 from here. Product updates
The most current version is 5.7.1
If fact you might not need to uninstall the version you now have installed. Just install that 5.7.1 version.
Do what ShootistBond007 said to do: remove the preview cache and remove the lock file.
The preview cache is somewhere entirely different from the LR programs to installing LR isn't going to change anything, whereas removing the preview cache may.
You were absolutely right! All that I had to do (after I deleted the preview file that you recommended) was to install the 5.7.1 update and everything is working smoothly!! I can't thank you enough - YOU ROCK!!!
You are welcome.