Force Shut down o n your computer or try to stop Lightroom in the "task manager" (ctrl+alt+del) once possible and restart Lightroom. Afterwards choose "optimize catalog" in Lightroom menu to prevent this problem in the future..hope this helps!
I have a similar problem after I installed the latest update today. Lightroom is frozen and does not find any pictures in any of the albums. There is just a spinning wheel. I restarted the app and the computer several times, but nothing helps!