Others have suggested using Boot Repair, and this may well work. If it doesn't, you could try my rEFInd boot manager, which eliminates several (mostly minor) GRUB-related problems when dealing with multiple distributions. rEFInd is also more reliable than GRUB at booting Windows. You can try rEFInd using a USB flash drive or CD-R (versions of both are available from its downloads page) risk-free, and if that works, install the rEFInd Debian package or PPA in Ubuntu.