Boot from an Ubuntu live cd/usb. You should be able to access what was your Ubuntu partition from this so that you can back up any important files (I must stress this: please back up your files for your own sake). Then, install and run boot repair:
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update && sudo apt-get install boot-repair && gksudo boot-repair
This code adds the Boot Repair repository, installs Boot Repair, and runs Boot Repair. Once Boot Repair is open, click the button that reads "Recommended repair (repairs most frequent problems)." Once it finishes, reboot.
If you still can't boot to your Ubuntu partition, then boot back onto the live cd/usb and run Boot Repair again, this time clicking Advanced Options. Check "Reinstall GRUB" if it is not already checked and check "Repair File Systems." Then click "GRUB Options." Check "Purge GRUB before reinstalling it" and "Purge kernels and reinstall last kernel." Click "Apply" to run the repairs. Follow all instructions given on-screen. After Boot Repair finishes, reboot. Hopefully you should be able to boot into Ubuntu again.
If even that doesn't work, then do a clean install of Ubuntu (This will overwrite your files, so again: back them up. Please.). On the screen that gives you the option of overwriting all other operating systems or installing alongside, go to "Something Else" and select the partitions (root and swap [just look for the one with an ext4 file system for the root, and the one labeled "swap" for the swap]) that you were using for Ubuntu previously and tell the installer to use these partitions for the new installation. Unless you want to get rid of Windows, then you can just select to overwrite.