The services could need to be reprovisioned. Try logging into My Account, mark the phone as lost or stolen, then undo the lost or stolen phone, then try restarting.
Or, the SIM could be improperly installed, try removing it and putting it back in.
Or the SIM is damaged and you might need to buy a new one. Try contacting a moderator to see if they can help by shipping a new one.
How was the phone unlocked originally?
I'm thinking the phone was dropped and the SIM got mis-aligned and would just require a re-seating of the SIM.
If you paid for a cheapie service; sometimes, iPhones can get re-locked by the original carrier.