Oldfred's comment is valid, but requires elaboration. Before going further, you should know your boot mode (BIOS vs. EFI). If you don't know whether your computer is booting in BIOS mode or in EFI mode, look for a directory called
/sys/firmware/efi in Ubuntu. If that directory is present, you've booted in EFI mode; and if it's absent, you've booted in BIOS mode. See this page of mine for more on this subject. Once you know your boot mode you can begin considering solutions:
- BIOS Mode -- If your OSes were both installed in BIOS, your options are limited, but the details depend on your disk and boot loader configurations:
- One Disk -- If you have one disk, then only one OS's first-stage boot loader resides in the Master Boot Record (MBR; that is, the first sector) of the disk, which controls the boot process. If you're booting straight to GRUB, then GRUB's first stage is presumably on the MBR (although there are some rare alternative configurations). Thus, you'll need to restore the Windows boot loader to the MBR to boot it directly, then restore GRUB. I'm not a Windows expert, but typing
BOOTREC /FIXMBR in a Windows Command Prompt window should put the Windows boot loader on the MBR. See this page for basic information on this command, and ask on a Windows forum if you need more advice. The easiest way to restore GRUB is to use Boot Repair from an Ubuntu emergency disk. Another way to restore GRUB is to back it up before you begin (with
sudo dd if=/dev/sda of=~/sda.mbr bs=440 count=1) and then restore it when you're done (with
sudo if=/path/to/sda.mbr of=/dev/sda); however, this approach runs a small risk of serious damage if you make a mistake, and you'll need to be familiar enough with your emergency disk (used for the restore) to locate the
sda.mbr backup file. Most people are better served by using Boot Repair.
- Multiple Disks -- If your computer has multiple disks, it might boot in much the same way as on a single-disk system; however, it's also possible to put the Windows boot loader on the first disk's MBR and GRUB on the second disk's MBR. If this is your configuration, then you may be able to boot to Windows without using GRUB by pulling up your computer's built-in boot manager. You typically do this by pressing Esc, Enter, or a function key just after you power on the computer (before the GRUB menu appears). This is the same tool you probably used to boot from the Ubuntu installation medium. It should show options to boot from each of your hard disks, and selecting the Windows disk should boot Windows.
- EFI Mode -- If your OSes were installed in EFI mode, then the firmware's built-in boot manager is a more sophisticated variant of the one I just described for the BIOS multi-disk configuration. This menu should have an option that's called "Windows Boot Manager." Select it to boot Windows directly rather than going through GRUB. This will be a one-time boot; when you next reboot, GRUB should appear. Again, this will work only if you installed in EFI mode.
- Mixed Mode -- Although rare, mixed-mode configurations are possible. In such a setup, Windows is installed in EFI mode and Ubuntu in BIOS mode or vice-versa. The fact that you're booting Windows through GRUB implies that this is not how your system is configured; I'm mentioning it only for completeness or in case somebody else in the future reads this answer and has such a setup. In this case, the firmware's built-in boot manager is likely to enable a one-time boot to Windows without involving GRUB. If Windows is installed in EFI mode, the relevant entry is probably called "Windows Boot Manager"; and if Windows is in BIOS mode, it's likely named after the disk on which Windows is installed.
In most of the preceding cases, using the computer's built-in boot manager will get the job done. Unfortunately, there's no standardization about how to get to this tool. If you don't know how to access it, you may need to consult your computer's manuals or just try rebooting. Sometimes you'll see a prompt, like
F10 - Setup, at some point during the boot process. If not, try hitting random keys -- it's usually something in the F8 to F12 range, but it could easily be something else.
If you've got a one-disk BIOS-mode installation, your task is harder, since you'll need to temporarily replace GRUB, as noted earlier, in the MBR. Be sure to read the pages to which I linked above, in the "One Disk" bullet point, and if you have any questions, ask them before you start digging a hole for yourself.