Have you been dual booting successfully for some time?
Were there any changes made immediately prior to this problem with hardware/software or updates?
An easy way to get detailed information on your drives/partitions and boot files which would be helpful is to run the bootinfoscript. If you haven't resolved this problem, go to the site below and read the instructions, download and run the bootinfoscript. It needs to be run on Linux, you can use a Live CD if you can't boot Ubuntu. Review and/or post the output, a results.txt file.