Inspiron 3668. Windows logo Key>Power>Restart. I see the whirling circle and "Restart." It does that for 35 minutes. I  try Esc, F12, finally is desperation Alt-Ctrl-Del. Nothing. I re-boot. It comes up fine. I suppose I should be glad that, apparently, everything seems to be intact. But I'd like to understand what happened. 

Any ideas?

TIA. SweetT