External Photo Library won't open in High Sierra

I have a MacBook Air. I moved my Photo Library to an external hard drive awhile ago to save space. I never had a problem opening it but don't remember if I tried to during the last update. Now It says "Photos cannot be opened because of a problem. Check with developer to make sure Photos works with this version of macOS.You may need to reinstall the application. Be sure to install any available updates for the applications and macOS." I am not at all tech literate. Hoping for baby steps to fix problem. Thanks.>