Quit Lightroom. Hold down the ctrl key (on win) or the Option key (on Mac), and start Lightroom again.
This will bring up a dialog that allows you to select a catalog. Depending on your usage history with Lightroom, you may see a list of catalogs in the dialog to choose from, or you can click the "Choose a Different Catalog" button to navigate to your catalog. Select a valid catalog (it will be a file with a .lrcat extension), and open.