Sorry, but you need to give more info than that. I'll assume that WiFi is enabled on the iPod Touch, and that you have a home wireless router that you are trying to connect to.
Things we need to know is:
1) have you been able to connect before?
2) what error message are you getting?
3) is the wifi router working for any other wireless devices?
4) do you have a firewall or any other security on the router that might have changed recently?
5) have you tried to connect to other wifi signals with any success?