First run GParted to be sure you haven't accidently overwritten your Windows partition. GParted is usually removed as part of cleanup during a fresh Ubuntu install so you'll probably have to download and reinstall it. The Windows partition will most likely be /dev/sda2. It will be formatted ntfs and size should be pretty close to what you specified when you installed Ubunt.
If the Windows partition looks okay your best bet is probably to use Boot Repair. You can find the documentation and instructions on how to use it
hereHTH. Good Luck!