iPhone Problems downgrading my baseband and stuck after a factory unlock!

I need some help here. I have a 3gs that was previously jailbroken and unlocked, I was running ersion 4.2.1 8C148a and modem firmware 06.15.00. (of course my gps was not working). I decided to do a factory unlock of my phone so I could update my version. I did the restore option through my itunes, all went well. When my phone turned back on it was in the configure screen and it will not get a signal using my sim card. I hooked up to itunes to try and configure and it says "no sim card installed". I then read the emails above and decided I needed to try and down my baseband to a newer base band. So i went through several steps that I could find on the internet. I downloaded Iphone2,1_4.2.1_8C148a_restore and using redsnow 0.9.15b3 I tried to downgrade the baseband. I choose the option of Extras- choose ipsw, then choose the one I just mentioned. I then went to jailbreak and went through the process. My redsnow gives me the done signal, however my iphone gets stuck on a part asking me to power back on to normal operating and then do a controlled power off, "slide to power off". I have tried this several times and it always gives me the same message. So now I am stuck with my iphone in configuration mode, not recongizing a sim card by itunes and unable to get it to downgrade the base band. What should I do now? Can someone help me?