The following sequence occurred with my Lightroom software:
I suspect that the software did not close properly when I turned off the computer. But I am not sure what to do.
Go to your folder containing the catalog. It is usually in Pictures by default. If you see a file for your named calalog ending .lock you should delete it. Do not touch the .lrcacat file or preview folders. Only delete the lock file. Then try re-launching Lightroom.
Thanks much. That fixed my problem. LR started right up!
Glad to help.