Same thing happened to 2 of mine. Both back up after constant self reboots all night. (Blinking light) 1st and 2nd gen 4Ks. My 3rd gen didn’t have this issue.

I just had 2 units, just now, brick at the same time. One mid show, the other while I trying to trouble shoot the first. Also 1st and 2nd gen. My 3rd gen is still working. Came here as soon as it happened. Heading to the Apple Store tomorrow.