[ubuntu] Mac Mini 6,3 12.04, working fine, then update borks system

I'm running 12.04 on a mac mini server, and all was well until an update in the pat few days. Ran a regular kernel update (whatever came out this week 6/3) and restarted the computer. Now Ubuntu won't boot, can't even get to grub. I just get a blinking cursor.

Any tips on how to diagnose and fix?

The hardware is fine, it works in OS X.