If you had windows 10, upgraded from 8, then when you install it and then verify with microsoft, no charge should be given for it. If you get in a bind for something to use till can fix the problem with microsoft, boot to a LIVE DVD of Linux Mint, or ubuntu MAte, or Zorin OS and use it in interim.
I went through the free win 10 period, installed win 10 over 8.1 and have been using it for quite a while. Yesterday, it crashed and after trying to get it going for a while, I reinstalled win 8 ( which I had my original dvd). Now it won't let me reinstall win 8.1 without buying it and I can't reinstall win 10 without paying $119. Can anyone help me dig out of this mess?