Hello Cameron2008,
If your iMac connects to that wireless network, you should be able to view the password in your Keychain Access on the iMac.
Open Keychain Access from your Utilities folder (Applications/Utilities).
Locate your wireless network in the list of items.
Double-click it and check the "Show Password" box.
You'll probably be prompted for your iMac's Administrator password to reveal the password.
If your having issues with Keychain Access, check this article for information on troubleshooting Keychain Access:
http://support.apple.com/kb/HT1060
Hope that helps and you haven't reset your router yet,
Charles H.