*Edit. I got it fixed! More information at the bottom.
I did a clean install of Ubuntu on a separate SSD from Windows.
After the installation Windows won't boot. I have tried removing all other drives from my computer and booting up with only the Windows drive. After this I'm greeted with the following message "reboot and select proper boot device".
I'm suspecting that installing Ubuntu has somehow also affected the Windows drive.
Is there any way to fix this, or am I doomed to reinstall Windows?
I had Ubuntu installed in Legacy mode (while W10 in UEFI). Ubuntu had installed the bootloader on the same SSD as Windows 10 for some reason (even though Ubuntu is installed on a separate disk).
I'm glad I did not do a clean install of Ubuntu in UEFI as this would've left the problem remaining on the windows drive.