Use Finder to open the folder that contains your catalog. Look for a file with the lock extension. It will have the same name as your catalog, but with different extension. That lock file needs to be deleted, and then Lightroom should start normally. This file normally gets deleted automatically when Lightroom closes. But if something abnormal occurs during Lightroom close down the file can be left behind and must be deleted manually.
Thank you very much for your help, Jim!