In your operating system, go to the folder where your Lightroom catalog is located. In that folder, you will find a file whose name ends with .LOCK (might be lower case). Delete the .LOCK file. Do not delete anything else.
Re-re-installing LR now. I will try this as soon as I can. Thank you~
You are brilliant!!! It worked!!! What causes this?
This is caused by an abnormal shutdown of Lightroom. (Crash, forced-close, etc)
When LR starts, it creates the LOCK file to prevent other instances from opening the same catalog.
When LR exits correctly, it deletes the LOCK file automatically.