If your phone, iPad or Tablet is lost / stolen, then please do the following:
1. Call AT&T on 1800 901 9878 from any phone. You will be asked to enter your USA mobile number for the sim card. Enter it. Then you will be asked "What would you like to do?" Say "More Options". You will then be asked "What would you like to do?" Say "Customer Service". You'll then be asked for more information. Say "Customer Service". You will then be transferred to an AT&T operator. Note that AT&T operators are only available Mon-Fri 7am-10pm, and Sat-Sun 9am-7pm (Pacific Daylight Time - Los Angeles Time).
A thief won't be able to run up big bills on your sim card - that's because it's pre-paid, so if they use up all the credit, then it just stops working. So there isn't any real rush to cancel your sim card (unless of course you added extra credit to your sim card - you don't want the thief to use up that credit).
2. If you are going to use a new phone, iPad or Tablet now, then simply visit an AT&T store, explain that your phone was stolen, you have an active "GoPhone" account and require a new sim card. The AT&T store will supply you with a new free sim card and copy over your plan, credit and any add-ons.
You can find your closest AT&T store here: www.att.com/maps/store-locator.html