Boot options might not appear if GRUB is convinced there is only one bootable drive. You can force the GRUB menu by holding Shift immediately after turning on your PC.
You can run this command to re-generate your GRUB boot menu and it will attempt to find any other operating systems:
sudo update-grub
You can run this commands to see your disks:
gnome-disks
This will give you options to mount your existing Windows disk or determine why it's not being detected. Sometimes if Windows was not shutdown properly it sets a flag saying it needs to be "checked" or "repaired" in which case Linux will not touch it. You can have Linux do the repair if you do this:
sudo apt install ntfs-3g
sudo ntfsfix /some/disk
In this case /some/disk is a disk path like /dev/sdXXX - this varies between system and you should take caution to not "fix" the wrong disk. If the package ntfs-3g doesn't exist try installing ntfsprogs instead.
