Chains on these things are loose and rattle, some people think it's normal...I don't.......Welcome!
Sorry to hear it. I don't know. Part of me would want to put the chain back on, and run it to see. Another part would want to go ahead and rebuild it, knowing that even if this just happened, something has been compromised. You could always get the oil analyzed, and see where the particles came from. It's possible that they represent manufacturing debris or normal wear and tear, and that the chain just recently came off. Or, you find out that it needs to be rebuilt.
I'd definitely want to know why the chain was off, and if it would do it again.