Mac tips
Lessons from my own wanderings
Hope this is of help. No guarantees, as usual.
Photos app won't open
When my Photos app on macOS 10.13.6 no longer opened on screen, I first used the standard Apple Support advice to hold down Command + Option (alt) while opening Photos. That stopped at 5% and did not advance even after 24 hours. So I tried to find the cause, but you don't have to even look inside the Photos database package contents. After a couple of weeks I noticed this always happened after I switched the OS Language setting _without restarting_. 
So here's the trick: do not ignore the "Restart Now or Later" dialog. After changing your Mac's display Language, always restart. Then, open Photos and presto!