active drive in win 10 is the boot partition... redirecting it to C means the PC isn't looking in the right partition to boot
On another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB (its just a boot disk)
Do you know how to change the active partition back to right one?
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
type
diskpart and press enter
type
list disk and press enter
this shows all drives available, DVD/USB and hdd, make note of hdd number
type
list vol and press enter
this will list all the partitions, the system partition is the boot partition (this is what should be active)
can you show a screen shot?
this shows how to set active part in win 8 using command prompt:
https://neosmart.net/wiki/set-partition-as-active/#Set_partition_as_active_on_Windows_8 (Win 10 should be the same)
If this doesn't work, you might be able to run a reset as i think it rebuilds bcd
boot from installer
on screen after languages, choose repair this pc, not install.
choose troubleshoot
choose reset this PC
choose keep files/settings (Files = library folders. Library folders = Documents, pictures, movies, music (the default folders that come with windows). Settings = logins and desktop)
Other choice is wipe everything which reinstalls win 10 as if its brand new
either way, PC will restart and reinstall win 10 from new. You lose all your programs on C drive, if you had multiple drives everything on other drives is left as is...