Constant reboot

[Anne Wilson]

Since I ended up with a large number of zombie processes for chromium-browser 
I decided to shut down the laptop completely, to get a clean, cold boot.  
There I came unstuck.  This is F12, fully updated.  The bubble fills up, I 
see the Fedora logo, then it reboots.  I could try to catch it and get a 
level 3 login, of course, but since I haven't a clue what's causing this, I 
wouldn't know what to do next.  Any ideas?

Anne