Server 2008 R2 Hyper-V: 'hypervisor' driver not installed
In the spirit of Don’t Be Afraid to Use The Science, I just tried this, by dual-booting away from Windows Server 2008 R2, and it looks like when you install the other OS, it goes and disables a few of the extra flags in the bcd store.
When you're in 2008 R2, run the following from an elevated command prompt:
bcdedit /set hypervisorlaunchtype auto
This should put the Hyper-V driver back into the launch procedure and you should be able to start your VM's again.