en0c wasn't very helpful.
blacksn0w is for unlocking the phone to be used on any GSM service provider. As you have a 3G and have just decided to jailbreak, I'm going to assume you have been using your iPhone just fine until this point?
If that's the case, I'd restore as a NEW phone, re-jailbreak and try it again to see if you have service.
Again, blacksn0w is ONLY if you want to use the iPhone on a service provider that doesn't sell the iPhone.