Got a SIM card two years ago for a month, can I use it?

Two years ago on my trip to the U.S., I bought a pre-paid card from AT&T. Now pre-paid cards work quite differently where I live - you put some money on it and can use it for years unless the money is used up on services. But with AT&T, I remember having to pay per month of service, so I think I just charged it with such an amount and thus it probably was active only for a month, right? I am coming back to U.S. after two years, can I somehow reload the card and continue without having to go to a store again? I tried to log in to prepaid services on AT&T website, but it tells me that my number "is not associated with a wireless service", which I am not sure how to interpret. Has the SIM been already cancelled or something?