I recently bought a used 3GS that had the 4.1 firmware and 6.15 BB on it. Without thinking, I upgraded to 4.2.1 without saving the SHSH. Now I'm on 4.2.1 with the 6.15 BB and I have the tethered jailbreak and ultrasn0w 1.2 installed. However, I get "No Service" when I enter my T-Mobile SIM card. I've tried everything from Airplane mode to Resetting Network Settings. Can anybody please help me?