The question mark (?) implies your HD does not have the system files (or Blessed as the booting volume).
Do you have the grey recovery CD/DVD that came with the system? If you do try restarting your system running off of it. Follow this Apple TN on the [http://support.apple.com/kb/HT1533|Startup key combinations for Intel-based Macs].
If you don't have the CD/DVD disks, you can also try starting up your mini in Target mode and connecting it via a FireWire cable to a second Mac. To try to setup your HD that way.
Lastly make sure you're using the correct OS-X version for your system.