I have installed several Windows 7 and 10 version on several of my Macs using Apple's Boot Camp feature.
Recently, I found that almost all of them have disappeared: I was not able to boot from them any more when I held down the option (alt, ⌥) key at startup - the Windows partitions would either not appear at all or not boot up.
The main reason in my case was that the MBR was reset to a plain GUID entry, and my Windows versions do not like that, because they cannot handle the EFI / GUID partition info that the Mac prefers. Why that even happened? Probably from repartitioning operations I frequently perform on my disks - and Apple's Disk Utility is quite ignorant of the needs to keep Windows bootable in this regard.
To fix that, I had to edit the MBR partition info again, in order to make the Windows NTFS visible again to the Windows systems.
In short, I used iBored to edit the partition layout of each disk that contains a BootCamp partition from something like this:
Note that this reduces the size of the first partition (you could as well change its size to the minimum, which is 33), and adds a new partition with the start and site matching what you can inquire using the Partitions window (see Disk menu):
using BootCamp on a non-startup disk).
Let me know in the comments or by email if you find this interesting and need more instructions, and I'll see if I can improve this article.