Sorry you're running into this. When a project suddenly loses assets and then throws the 'Oops, something went wrong' message, it often points to a momentary glitch or a possible file corruption.
Could you let me know which browser and OS you're using, and whether this issue is happening with just this one project or others too? Also, if you remember the last action you took before the error popped up, that could help us narrow it down further.
In the meantime, try logging out and back into your account, then reopening the project. If that doesn't work, check if there's an autosave or earlier version available under your project history.
For now, it's happening only in this project. I tried logging out and back & also tried secret browser as well, still not working.
To elaborate what happened...
I ungrouped the objects > some of the objects disappeared > clicked cmd+z to restore > the error popped up > now I can't click anything else other than the modal
Also consider clearing your cache. I do find that Project Neo needs a short break around 30mins, especiall with bigger projects.
What I mean is close your project and then reopen it after a few minutes. I have also found turning my computer off and on again has helped. Mind you this could also be to to a RAM issue on the machine.