How long did you keep the 'Reset' button pressed down? It needs to be for at least 10 seconds for it to work. (Resetting the router takes it back to the way that it was when it left the factory, which will probably remove the need for a security key altogether. If you can get that far we can then tell you how to put a new security key onto your router).
Alternatively try a call to TalkTalk, as they should have a record of the security key that your router was sent out with:
0345 172 0088 (Free from your landline if your phone service is with TalkTalk. Charged at the same rate as normal 01 or 02 numbers, which might make it free if you've got an 'inclusive package', with other providers).