I have seen possibly 5 duplicates on my searches for this, and haven't found it. Is there one, and if not could one be created?
Windows missing from the GRUB menu depends of many variables:
- Windows XP, Vista, 7, 8? (is it even installed?)
- UEFI vs BIOS
- Wubi (this apparently also affects)
- Status of the partition
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.
I just ran into this again and I had to spend more time and effort than I or someone else probably wants to find a good answer.
The best answer I could find so far is this one on GRUB does not detect Windows. This should be a extended to signpost answer covering typical cases why
os-prober is unable to find any installed version of Windows (or any other operating system).
Braiam's approach was to hope that the system will heal itself. From my point of view this hasn't shown any progress since then, it has failed and needs our support to find a good answer to this question.