Entities gone

No clue the details or what happened, but my pets and other entities (a zombie, villagers, lots of cats, lots of dogs, a pig, some cows, some sheep, lots of fish, some axolotls, minecart chests, iron golems, slimes) had all vanished.

They were all there, I left for like an hour to get nether quartz in the nether, came back noticed it was eerily quiet and all of them were gone, thankfully I had a backup but that was really scary many of them have been there for so many years and I really don't want to lose them. I'm kind of nervous to play because I have zero clue what might've caused it.