This answer assumes your computer boots via EFI, as do most computers that shipped with Windows 8. If you installed Windows 8 on an older computer and it boots in BIOS mode, Rick Dunn's answer is likely to be more helpful.
Most computers include a built-in boot manager, which you can access by pressing Esc or a function key soon after booting. Unfortunately, there's no standardization in how to get to this menu, so I can't be more precise about it. I recommend you check your documentation or early boot messages for clues, or just keep rebooting and hitting function keys until you find it. With the menu found, you should be able to boot Windows.
From there, how you proceed depends on what you want to do:
- If you want to try installing Linux again, you can just do so. (In fact, you could skip booting into Windows first and just re-run the Linux installer.)
- If you want to remove GRUB, try opening an Administrator Command Prompt window and typing
mountvol S: /S. This mounts the EFI System Partition (ESP) as
S:. You can then use the Command Prompt window to delete the
S:\EFI\ubuntu directory. Thereafter, you shouldn't see the GRUB prompt again.