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.