screen flashes a file with a? on a white screen

Thats a good sign! you now know you put things back together correctly. But, the HD doesn't have any files on it at this point - Correct? You'll need to partition the drive & install the correct OS on the system using your OS-X CD/DVD that either came with your system or bought.

If you still have your old drive and it's working you could try migrating your files back onto this drive. To do this in your case you'll need a second Mac system to be the Master which will host your old HD via an external FireWire HD case iFixIt FW/USB kit or a USB HD case iFixIt USB kit which is only a connection kit Vs case and is much slower, but cheaper.

Once you get your old drive patched up with the needed hardware connect it to your friends Mac and check it out with Drive Utility. Don't do repairs! Unless you have a backup. Setup the connection FireWire (best), Ethernet or WiFi between your systems (review this Apple TN: Migration Assistant) Setting your system up in Target mode (slave) copy over your files from your old HD.

If all goes well you should have a booting MacBook with all of your files - Good Luck