what stage of install did it stop at?
how long ago did you upgrade to win 10? I assume you upgraded from ultimate so if its in last 3 or so weeks, from the desktop in win 10, go to all settings/update and security/recovery/go back to previous version and it should put Ultimate back on PC for you.
If you upgraded over a month ago, your ultimate licence is now a Win 10 licence and you would need to get a new licence to install win 7.