Windows missing from the GRUB menu depends of many variables:
- Windows XP, Vista, 7, 8?
- UEFI vs BIOS
- Wubi (this apparently also affects)
- Status of the partition
- etc.
A single question cannot ever be canonical for all those possibles variants of the same problem, instead focus in doing several questions that are clear enough with all the necessary information to diagnostic the problem and give the proper solution and the sensible solution.