Lost content of slideshow in Photos
Now up to Big Sur 11.5.2 upgrades in Macbook Air.
Made a slideshow in 2017 and stored under 'My Projects' in Photos. Had viewed it over the years but not recently. Several upgrades later now, I went to view it and the icon for the 'slideshow' is there with the label I put with it but--- when I open it and hit 'play', all that happens is the music I put with it plays but there are no pictures. I don't know what happened or how to recover it. I went into my USB's that I store things in and it (the slideshow) isn't there either.
The pictures used are no longer in my Photos in my Mac they are stored on USBs.
Any ideas what happened and if I can recover it or is it a 'do over' and if so, how do I keep this from happening again?
MacBook Air 13″, macOS 11.5