I assume you mean you have a pre-paid SIM. There is no roaming with the ATT Pre-Paid GoPhone in Canada so it would not be able to register on the network while your up there. ATT only has roaming agreement with the Post-Paid service in Canada.
When you get to the US you should be fine if I remember correctly. If you still have issue just stop into an ATT store and they should be able to help you get going.