Go to the location where your catalog is saved and get rid of .lock file (do not delete other files) and then, launch Lightroom.
tthanks for contacting me Chetna. I seem to have several catalogues but was able to delete one that was locked but there is another on a thumb drive that does not allow. Also I have not been able to back up for a couple of days.
just was able to unlock another one and now it is checking the integrity of the catalog. Is this also associated with the backup issue?
It means it is checking for corruption in that catalog.
If there is no corruption, it should probably work fine after checking integrity.