Issue with AT&T Prepaid Service

I purchased a prepaid SIM card from Best Buy, which I activated online.
As I didn't have time to enter the SIM card while I was in the US, I had to start it from Mexico.
I received the number, activated the account, and paid successfully.
Because it was roamed, I was able to utilize it as Telcel. (But at a very sluggish speed)
I returned to the States today and discovered I could not access the AT&T network.
It just reads, "No service." I could use the roaming signal when I return to Mexico.
What ought I do?