Sorry Llyffant, I can't offer any solution but my posting will raise your post up and maybe draw more attention.
In the meantime (and I am no expert), have you downloaded/installed a new programme recently or visited a website that maybe caused the machine to restart? In my own experience (which is not professional) this sounds like a dodgy programme or a bug from online.
Don't get scarred though, I speak only from experience which is limited to me alone, mostly.
I hope you get a speedy reponse because this is standard stuff for techno bods and there are many on here :thumbsup: