1st try opening a terminal in Ubuntu and see if we can't fix grub, the boot manager.
type sudo update-grub
give your password
it will report what it did, see if windows is there now or reboot.
If that didn't work try installing boot-repair in ubuntu https://help.ubuntu.com/community/Boot-Repair
Try the default repair and if that doesn't work under advanced options you can restore MBR.