what does it mean when your sim card is "not found" in a t-mobile sidekick?

[that one cool guy]

I just recently bought a prepaid sim card over the phone with t-mobile. I called them up and they activated my sim card and gave me my new phone number. when the sim card is in, my sidekick says, "SIM card not found" (code SIM1) somebody please help me. I really need this phone on!!!