Do you have an nVidia graphics card?
Hmm, yes I do...
Prompted by your question I've googled and see that this may well be the problem. I did install it at around the time this problem arose. Ironically I installed it (GTX 970) to try out DaVinci Resolve instead of Premiere Elements!
It would be convenient to have Premiere Elements working for quick edits, but not so convenient that I want to buy an upgrade to a later version of Elements. Are there any workarounds I can try to get it working with this Nvidia card in place?
Wow, I was sceptical that a driver update would fix it as I had installed up-to-date drivers on this old card when I installed it in November, and even more sceptical that a games-specific hotfix would make any difference.
But I just installed the latest (14 December) drivers and Premiere Elements 12 now loads and opens a new project!
Thank you for your help with this.