Could be one of many things - nearly all starting with the router configuration - what encryption are you using (WPA/WEP)? what band (2.45Ghz)? are there specific access-restrictions configured (mac filtering etc)?
Do you see the SSID of your network when you select the WiFi page in the iPad settings? (if not, is the router set to not broadcast it?) - or are you seeing any specific error messages when you try and connect?
Finally, are you able to connect to any other WiFi networks (if available)?
My iPad (not unique to mine I'd imagine) has an annoying habit of not automatically connecting to any wifi networks at times - I actually need to go in and select it from the list in the settings.