How do I cap off a pipe below by hot water heater's cutoff valve?
Absent the presence of any valves...go to hardware store plumbing dept. And get a 3/4&quot; sharkbite or tectite end stop. You just shove this on to the end of the (cut or removed)  blue pex pipe. Get a 1/2&quot; one in case I'm wrong judging size from the pic...be sure to buy the same size removal tool as well. It is cheap.
I keep these in my emergency preparedness kit...
If you post a pic of the top of this valve, we can help with temporary plumbing. Good luck with your recovery!