Windows 10 no events no update history, virus?

Made up a new windows 10 computer 2 months ago after last computer got infected by cryptolocker by inexperienced user. New computer has been in use daily since without problem, however was left on for 2 days around 2 days ago, which at 3am seems to have reinstalled windows..?

A new windows.old folder now exists, old programs are still installed but some - including avast - were not working correctly (all working now), system settings like password to login have now retued, there is now no event logs or update history prior to 2 days ago. sfc fine. No obvious viruses.

I'm not sure what's going on, is it likely reinfected? What can I check for further?