I tried updating my iPhone, but now it says "SIM is invalid" I have an iPhone 3GS 32GB. I have just tried updating the phone using iTunes. The phone is second hand, and was unlocked when I bought it. I didn't know that when you try to update it, it loses everything. Now it isn't reading my SIM, even though I've used this same SIM in the phone before I tried updating it. Now, when I turn the phone on, it comes up with:


  SIM not valid - Only compatible SIM cards from a supported carrier may be used to activate iPhone. Please insert the SIM card that came with your iPhone or visit a supported carrier's store to receive a replacement SIM card.


What does this mean? And how can I get my phone working again?
This means that the phone was software unlocked (which requires a jailbreak). Because you updated the device using iTunes, you have removed the jailbreak and the phone is no longer in a state of carrier unlock. This is actually common practice and a great number of people end up getting burned (they either just don't know better or are lied to and told the phone is factory unlocked).

You can call the carrier the phone is originally locked to and have them unlocked it (they may charge a modest fee or if you luck out, do it for free as that particular model no longer being manufactured) or re-jailbreak the device and software unlock it yourself, provided you have a baseband revision of 06.15.00 or older.

Note: Apple can't help in these matters.