os-prober is called by grub-mkconfig to find non-Linux installations. It is failing to find your Windows system, which is why it is not included in the GRUB menu.
It appears that your Windows setup has been damaged, this may be as simple as the boot flag not being set on its partiton. You may need to boot your Windows rescue disc/partition and run fixmbr - or whatever it's called for your Windows version - before booting a live CD and reinstalling GRUB.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)