[ubuntu] grub gone

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http://ubuntuforums.org – Hello, I think I seriously screwed up. I reinstalled windows, because AMD drivers messed up my registry beyond repair. When I restarted the computer after the Windows configuration was finished, I freaked out, grub is completely gone and Windows boots itself automatically. What did I do? I know that the Ubuntu partitions were untouched, so the OS must be fine, right? I really hope I didn't seriously screw something up :( Thanks for any help. :/ (Hardware)