I recently restored my iPhone 3GS?

It was having problems with it constantly looping at the apple logo, before the issue the sim card for the phone (t-mobile sim card) so I restored the iPhone back to it's default settings since I had no choice but to do that, (Mind you this is my father's boss phone given to him but we had no choice but to restore given all other options didn't work). Now the iPhone is saying"The SIM card inserted in this iPhone does not appear to be supported" when before it was working no problem

Any help appreciated.

When you restore a iPhone you need a sim card to reactivate it like when you first get the phone new. You can just put in the sim to activate and take out afterward. There's nothing wrong with the phone just need to be activate with a sim card. The phone is likely AT&T if you are in the US so just get one from a friend or AT&T store.