I too had this problem. Don't know what happened but the link between the files and the program stopped working (was fine after upgrade to 11). I tried all of the suggested items below that I could make sense of but none worked for me.
I was at the last thing I could think of, uninstall the program and re-install it. Unfortunately it appears that Microsoft will not let you uninstall this app! It is grayed out!
But in trying to do this I found the fix.
- Open Settings from the Window menu
- Select Apps (5th one down) in the left hand menu
- Select Apps & Features (first one) on right hand menu
- Do a search or scroll down until you find "Microsoft Photos"
- Click the 3 dot menu for this item and select Advanced Options
- Scroll down until you see the option "Reset"
- There are two options under this; buttons to "Repair" and to "Reset"
Clicking the "Repair" button did the trick for me and I can not double click a photo file and it will open in this app.