I thought I had disabled Win10 updates. I find them to be worse than viruses and have been pestered about taking an update on a daily basis. I keep saying "no" only because there is no "hell no, over my dead body" button.
Come back from coffee this morning and it's rebooted without my permission and loading an update.
Sure enough, it won't reboot properly with errors and can't load explorer.exe.
Command prompt to system restore and it says no restore points. Reboot into 'safe' mode and I managed to get it to restore previous (yesterday's) version.