Check with the network you have not been barred.
In the UK you can get barred if you report the handset stolen or you have unpaid bills but not sure if it is similar in the US?

I suffered this too!!!!

I could see other network providers when the phone searched but it told me I had no service whatsoever...there was no service!!! My sim was fine too and was functional in another phone. I tried retarting the phone (soft restart) and nothing...dead as a door nail everytime.

Basically I hard reset....and it worked. I have no answer for it at it. I assumed that the registry hacks I implemented two days before may have caused some form of corruption. Basically the hacks I implemented were in regard to ICS over BT....I wasnt able to use my carrier for Internet sharing over BT....So I edited out the other providers and removed the "forcedCellConnection" registry entry......It all worked. I used it a few times and had also turned the phone on and off since doing the tweaks....Basically the setup since going to WM6.1 was all working...then out of the blue in the car...no service and definitely in a 3G area. I was trying to access the Internet at the time so I could use the yellow pages....

I've since set my phone bar the tweaks for ICS I used previously and no probs....I will try again though, I need ICS with my own provider and having an unlocked phone should enable this.

I tried everything to get it to work other than a hard reset....nothing, zilch, zero, diddly squat. I'm not barred....my bill was paid. Put it this way...I travelled over 200 kilometres that day...no service the whole time, And no network issues either, I checked that too....