Sorry for your lost, is Find My iPad on and configured before you lost it?
You know the iPad needs to be ON and have Internet connection for you to locate it.
If the above condition is true, and you still couldn't locate it then probably the thief has restore the iPad as new.
Report to the police and if it's a Wi-Fi + Cellular version, report to the carrier.
Change all the password that were used in the iPad.
Lost or Stolen