Very happy I did find my problem. It is kind of embarassing. The cause for the crashes was an addon that I updated 2 days ago. And these were hefty - can only reboot - crashes. Ever since disabling it, I am having no more problems.
MiniMap by Fyrakin was the addon that caused my problem, specifically version 2.32. At first I thought it was my PC, as its completely new and I've just returned to ESO. Have had the addon before and never any problems, which is why I never considered it. My screen would go black and I'd have a sound loop for a short time. After a bit of time I could at least reboot (but only through the power button).
Later on I saw http://forums.elderscrollsonline.com/en/discussion/comment/1779560#Comment_1779560
which suggests removing the addon.