The problem is probably due to the wrong partition being booted when the computer starts.
To fix this boot from a live CD
- Open system -> administration -> gparted partition editor
- Check which partition is flagged with boot
if the partition flagged with boot is ntfs (File System column) it probably means the computer boots into Windows without passing by grub. (If it is not marked with ntfs do not continue with my instructions)
To fix this right click on your the ext4 (File System column) (the Ubuntu partition) manage flags and select boot.
Restart your PC and you should be presented with the GRUB menu.
If your computer refuses to boot after proceeding with the above steps, reboot from the live CD and mark the lately flagged ntfs partition back with boot.