I seem to have the exact same problem that Phil here is facing ever since i updated to 16.04.
(Note: I did a fresh install of 16.04 over 15.10, not the upgrade from within 15.10). But there is a slight difference. If I try to graphically shutdown my laptop (the top panel button I mean) , it acts in a similar way to how Phil’s laptop does & I end up holding the power button to shut it down.
But if I type the command poweroff in the terminal , it seems to shutdown normally. I have been using the poweroff command ever since. And I am updating my system regularly. I will try some of the solutions posted here.
Model: Dell Vostro 3546
Intel Core i5-4210U , Nvidia 820M 2GB graphics , 4Gigs RAM.