A friend and myself were running around in the twilight forest and tried to fight the hydra. He got to close, she did a big attack, he was onehit and for some reason the grave either didn't spawn at all or it got destroyed by the hyrda.

Is there any way to get back the items via command? Are the items stored somewhere in a file to get them back?

Which version of the mod are you using? This sounds like an issue I've fixed.
To restore a grave, try /yigd restore [user] or /yigd moderate (and navigate to the grave you want to restore)

Sorry for not providing that.
We are using YIGD fabric 1.4.6 with fabric version 0.14.10 and twilight forest 4.2.310

Mc 1.19.2

Just an idea we got right now: Could this be a problem in combination with charm of keeping from twilightorest? We will test that later

Yeah charm of keeping is known to delete items it's supposed to keep. However I don't think it's an incompatibility due to me being able to reproduce it without yigd present

Well, that could be true, but the grave didn't even spawn or the hydra destroyed it. It was a bit too hectic to notice it.

You can use the command /yigd moderate and select a player, or use the command /yigd grave [user], then try and find the grave. Filter by destroyed, and if it's destroyed you should be able to find and destroy it there. If it's not there I'm sorry, but there's not really anything I can do

I'll close this issue as there's a chance that this wasn't an error from my side, and twilight has fixed the charm of keeping issue. Let me know if this would happen again however, and I will take a look at it again. Thanks for reporting it though!