Harmonic balancer comes off

Well it's good you didn't run it very long. I'm sure a good inspection of the Cotter key and also like some are saying making sure the outside didn't separate from the inside. That will tell its own tale. of course the nut holding everything on is that there? or was it just the outer ring slip? On my car just the outer ring went inwards and scraped the timing cover, luckily I caught it before it went through the timing cover or went the other way and flung out and did a lot of damage.