I guess you need to enter the pc BIOS and set uefi from there.
It might also be a Grub issue. Grub is GNU/Linux bootloader that is in charge of listing available oss on the system, if the other OS was not present when you installed GNU/Linux that might be the issue, Grub is unaware of its existence.
It is now probably easier to reinstall GNU/Linux (with other os drive attached) than edit Grub conf file.
For more info on how to do that, the web is full of guides.