why is first screen after Restart keeps re-opening?

My 2011 era Macbook Air uses 10.15.7 Catalina. Recently I tried a backup with my Seagate Backup plus that seemed to work, though at end it said it did not. While in the backup and since, the screen with the "island" that first appears after ReStart and requires the basic Password for the computer started to appear repeatedly. Even after Restart. Can you advise how to fix?