is it a blank screen with a flashing cursor?
On another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB << if you don't have it already (possibly just a boot disc)
windows might still be there and is just hidden from boot loader, try this:
change boot order so USB is first, hdd second
boot from installer
on screen after languages, choose repair this pc, not install.
choose troubleshoot
choose advanced
choose command prompt
We need to use the Bootrec.exe tool. Click on command prompt and type in the following commands, one after the other:
bootrec /RebuildBcd
bootrec /fixMbr
bootrec /fixboot
Exit
Now go ahead and reboot your system. In some cases you may need to run some additional commands.
bootsect /nt60 SYS or
bootsect /nt60 ALL
sometimes resets are too effective and delete windows so do a fresh install using that USB above
change boot order in BIOS so USB is first, hdd second
boot from installer
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and reactivate once finished
On the screen where you choose where to install win 10, if it gives you an error about GPT drives, delete all the partitions on the hdd and press next. If it still gives error, cancel out of the installer and restart PC and start installer again, it will accept next on that screen this time (some PC just need a restart here)