This normally happens after an abnormal shutdown of Lightroom (ie. a Crash).
In the directory where your catalog is stored will be a file with the same name as your catalog but with an extension of .LOCK. yourcatalogname.lock.
Rename this file to something else and then try to start Lightroom.
Thank you so much! This worked. You are right. My Computer Security program was freaking out and crashing that day and I was working on my pictures while I was doing online chats to fix my Security program problems. Since I posted this question I removed and reinstalled Lightroom and was shocked when that didn't fix the problem....all because that catalog with the LOCK extension was there.
I really appreciate your help - ALOT.
THANK YOU SOOOO MUCH!