This is an old problem and not just limited to Dell's, from what I find doing a search.  No specific hardware, although one I found was memory related. Glary utilities may also be to blame, or at least partially to blame.  We see a lot of problems caused by these types of problems and only real fix is a complete reinstall.  Avoid PC Repair and/or Registry Repair programs.  
There have been a few mentions of this issue with laptops or other dell desktops, but tried some of the suggestions in other threads (disconnecting power source, changing mother board battery, turning off secure boot, etc) and nothing.  Any help or reasons why I can't boot?  Sometimes it says it's running diagnostics, but it doesn't seem to do that and it just gets caught in a loop.  I downloaded a few updates before this happened, but only installed Glary utility update.  I left the room and the computer had rebooted itself.  When I got back in, is again rebooted itself and then I got the blue screen that the computer was having issues, so it would be a minute while they sent info and then I just went into the mespilock loop.
(a7) me fw downgrade - request mespilock failed 3668 inspiron