Since I came back from abroad, my phone has not been reading my SIM card. I thought it was the SIM card at first, but after trying it on other people's phones, it worked perfectly fine. After thinking it was my phone, I went to the Apple store, and they ran a diagnosis on my phone and told me that there appear to be no issues with my phone. However, what they found out is that my phone's serial number has been marked as blacklisted (stolen or lost). They told me that only my carrier can blacklist my phone, so I need to get it un-blacklisted asap.