Using your system browser, go to the folder that holds your catalog. You will find a file there with the .lock Extension. It will have the same name as your catalog. After you delete that file you will be able to start Lightroom again normally. That lock file is something used by your catalog, and sometimes it doesn't get deleted when Lightroom closes, so you have to do it manually.
I have no idea how to find the system browser nor the folder that holds my catalog. Could you explain how i would find that please?
If you are on a Mac the system browser is called Finder. If Windows, it's called Windows Explorer. If you don't know where your catalog is located (since I only know about Windows) I can only suggest that you use the search feature of your operating system to look for .lrcat and see if you can determine where it is located. Go to that folder using either Finder or Windows Explorer and delete the file that I have indicated.
i also have windows but still cannot figure out how to find this. Is it in my control panel?
Go to the start menu.
At the bottom of the start menu is a place where you can type.
In that typing place, start typing Windows Explorer.
The start menu will change and you will see Windows Explorer listed. Click on it.
When Windows Explorer is open, Double-click on the Pictures folder, then on the Lightroom folder.
In that Lightroom folder you should see a file that has the .lock extension.
You need to click on that file and delete it.
Then you can close Windows Explorer and Lightroom should start normally.
I googled it and figured it out.Thanks!
It would make life simpler for a lot of people if they understood what a good friend Google is.