In addition, now when I click on the catalog in my pictures file, a "Locked file appears, which I try to delete. I delete it but it comes back.
The lock file will always come back if you delete it while LR is running. Any time LR is open it creates that file so it cannot be opened more than once. When it is closed the file is normally deleted, but if LR is stopped not in the normal way, by a crash or an electricity outage or a force stop, it doesn't get a chance to do the delete. So close LR first and then open the My Pictures/Lightroom folder and look for the .lock folder. However, since you have been able to open LR and create a new file, I doubt you will find it. Now, LR still closed, find your old catalog file - Lightroom 4 Catalog.lrcat - and double-click on it. Does that cause LR to open? Is all your work there? If LR does not open, navigate to your most recent backup .lrcat file and double-click. If you haven't been backing up your catalog regularly and your work catalog is corrupted, you have probably lost your work.