2 SIM card error

I recently travelled out of the country and replaced the SIM card (fully loaded and I have used it on another occassion) in my phone. My phone keeps giving me an error saying that I can’t have two SIM cards at once. There is only room for one SIM card in the slot. I am not able to turn off/disable my old SIM card. In other words, my phone thinks i’m using two sim cards, and won’t let me get rid of one What can I do?