Hi,
as the connectors are only on one side of the cable, you shouldn't have damaged anything.
A second thing is, how did you put it back together? With the hard drive connected upside down it shouldn't fit in.
Check the hard drive cable, it needs to befitted well into the connector on the logic board as well as into the connector on the hard drive.
Use these button combinations to set your iPod into disk mode:
http://www.command-tab.com/2006/03/30/hidden-iPod-commands/
You might need a couple of tries to set the iPod into "disk mode"!
When in "disk mode" connect the iPod to iTunes and restore the iPod. After its restored you should be able to use your iPod as usual.
If the cable is seated well on both sides and you can't get it to work, you might have broken the hard drive cable.