Maybe you stumbled into a variant of this this mess. Get yourself a Debian installation CD ISO, copy it onto a USB stick, boot the installer, and choose Rescue mode (if not in the main menu you might find it in Advanced). In resce mode the installer will load some stuff and detect most of your systems settings. Don't get nervous. Nothing has happened so far. When it comes to grub, choose to reinstall grub and choose the correct device(s). Depending on your setup you might have to install grub into all MBRs (Raid). Reboot. If that fixed your issue, make sure to run sudo dpkg-reconfigure grub-pc to fix your debconf database entry.
Edit: Has also been reported as #984426. Judging from the maintainers mails this seems to be an incarnation of the report I linked above. Maybe your system bailed out during the upgrade as announced here, and you restarted it anyway? In any case, the installer's rescue mode should help you.
same issue here. Trying to repair now grub. What a mess 
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