I have had a working dual booting fedora (23) and Windows 10 system (for quite a while).
I was tinkering with an old computer and was using the above system to update a 32 bit Ubuntu system on a USB drive before using it on the older system, and Ubuntu asked to update its grub loader, so I said yes.
I had the option to "install grub" on each of the hard drives (the USB & the native HDD on the dual boot system), but I only installed it on the USB drive.
Now, I can still get the dual boot system to boot, but I HAVE to have the USB drive connected. I have tried a few fixes including "boot-repair" and "grub-update", but still no joy.
Any suggestions? Grub and bootloader documentation online seems to be spotty and obscure at best.