my 4s gets stuck at trying to activate my iMessage what is wrong? imessage worked fine until I had to do a system restore.

My iphone 4s was stolen Tuesday and I gave up hope on finding it. I set Find My iPhone to delete the phone as soon as it connected to the internet. I retrieved my phone yesterday and was unable to cancel the deletion of the phone. I restored it to the last time it was synced, but my imessage will no longer work. It worked perfectly before being erased. Also while my 4s was m.i.a. I got a new sim sim card to use my 4. My iphone 4 still recieves the imessages even though I switched the sim cards out and the 4s is stuck saying, "Waiting for activation." Please help!!!!