I had the exact same problem on an HP Pavilion G6-2240sa preloaded with Windows8. It turned out that for some reason that key files on the DVD I burned on the same machine were not recognized at boot time. The solution in the end was to create a USB stick boot drive by downloading the data from the Ubuntu site and creating the bootable drive using the software site recommended by the Ubuntu site "How to create a bootable USB stick on Windows". Do not just copy the files to the USB drive. I also had to edit the BIOS to look for the bootable USB drive before booting from the internal hard disk. If you would like more details with a blow-by-blow account see here
Did you install Windows after Ubuntu,if that so then use boot repair https://help.ubuntu.com/community/Boot-Repair . this is a really great tool. You just have to boot ubuntu through live cd and select try ubuntu and follow instruction given in above link.