If Ubuntu "forgets" about Windows in grub ...
If you have a long running Ubuntu-dual-boot-installation with some kind of Windows installed in parallel there might com a time, when grub suddenly "forgets" that there indeed is a Windows system to boot or screws up the config.
The two most common message here are "wubildr missing" and "drive map missing". If your installation does not show one of those you might try the simplest solution:
First make sure you have osprober installed, otherwise install it via
sudo apt-get install os-prober
After that rewrite your grub config by using
If that does not help or you're getting the "drive map missing" error you could give boot-repair a try, a tool present on the Ubuntu live CDs, but which can also be installed later by using this piece of code:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
If you specifically get the "wubildr missing" error you should try adding the wubildr to the Windows partition again before trying to repair everything with the hammer above. This is actually quite easy, since you just have to copy some files (the example expects the Windows partition to be sda1):
sudo mount /dev/sda1 /mnt
sudo cp /host/wubildr /mnt/wubildr
sudo umount /mnt