You will have to log into your Router and see if the Security Key is available for viewing.
If not, as you say, it would be easier to reset the Router to Factory Settings and start again.
We can not help you to find missing Security Keys:
http://support.microsoft.com/default.aspx/kb/189126
"Microsoft policy about lost or forgotten passwords"
Microsoft can not help you retrieve passwords of files and features in Microsoft products that are lost or forgotten.
Read the informationBANNING Password Cracking Tools information being supplied in these forums in the above Thread posted byBill Fill MSFT, Moderator
Cheers.
Mick Murphy - Microsoft Partner