? folder comes up on restart
Replaced new hard drive. restarted and ? mark on folder comes up
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 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.
connect an ethernet cable (if possible) and hold down the option key and r at the same time- if it doesn't work, Apple geniuses can help.
the option key on it's own brings up bootable storage devices connected to your mac - try that and see if anything comes up, it allows you to set up wi-fi with the efi firmware, too.