First, try disabling Secure Boot. If it's enabled, some versions of GRUB can't boot Windows. This is a GRUB bug.
If that doesn't help, try installing my rEFInd boot manager. You can try the USB flash drive or CD-R image without installing it, if you like; or dive in all the way by installing the Debian-package version in Ubuntu. (Note that the USB flash drive/CD-R version will boot Ubuntu correctly only if you do not have a separate
/boot partition or if you manually edit your kernel options.) rEFInd is less finicky about booting Windows than is GRUB, so it's more likely to work if GRUB gives you problems.
If you prefer to stick to GRUB, that may be possible, but you'll need to provide more details about what's not working, like any error messages you're getting, whether you see a Windows logo appear, etc.
Sometimes this problem can be caused by a missing Windows boot loader. (Typically you'll get some variety of "file not found" message, or rEFInd won't show a Windows option at all, if this is the case.) You'll need to either restore the Windows boot loader files from a backup or use a Windows emergency disc to re-install the files to fix this problem.