First, where is your Photos Library located? Is it on an external drive, or is it in the internal drive's Pictures folder? Or is it in the iCloud Drive folder? or where?
If it's on an external drove, how is this drive formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:
Move your Photos library to save space on your Mac - Apple Support
If this drive is in a an incompatible format, stop running Photos with it immediately! A Photos Library can sit on an incompatible drive, but running it may corrupt the database.
- If that's not the problem, then I wonder what has changed? Can you think of anything?
- Have you used any "cleaner" apps or "virus hunters?" These can corrupt the Photos Library. VPNs may interfere with the Library.
- It's great that you have the pictures on your iPad! Are you using iCloud Photos to synchronize your pictures? Do you use Optimize Storage? If you use iCloud Photos, do you see all your pictures at iCloud.com?
- Have you tried restarting the Mac?
- Have you tried restarting in Safe mode? This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
- If you're not using iCloud, then try a Photos Library Repair--close Photos and re-launch by option-command-clicking the app icon. If you do use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.
There are some more diagnostics we can try, but we need to know about these things. Let us know…
