Cold water valve in upstairs bathroom has burst loose and flooded the vanity and bathroom floor and is pouring downstairs into the kitchen. Why did it come loose and how do I fix it? I have shut off the valve under the sink and cleaned up so far.

It seems to me that stem got loose, you didn't notice until it unscrewed all the way... Make sure you reinstall correctly. Use crescent wrench to tighten the stem up. Milo

Milo thinks it's a loose stem while I tend to think it just might have been the 1/2" Chrome compression nut that came off/ Time to fill us in.
What came loose? The stem and handle or the entire valve? Back to you, Tom