was this during the original install of windows 10 ?
what is it doing now ?
it should be uninstalling the win 10 and re installing the previous version of windows.
but if it does not complete it in a few hours than it got frozen. you have no choice but to turn the computer off ( you can just hold the power button in for a few seconds if nothing else works )
then wait a minute
turn it back on and hope it straightens itself out one way or the other.
let me know whether it did or not. there are some things we can do if it doesnt but it depends on what state it gets to.