iPhone 4 won't send/recieve texts or calls...Please Help!!!!!

I recently bought a used (more than 1 yr old) iPhone 4 off craigslist. It worked great unitl this afternoon when I restarted it. When it turned back on, it just kept searching for bars but never found any. I decided to restore it with iTunes. Now it won't send texts or make calls. With calls, it shows the calling screen, and the person I'm calling picks up but my screen continues to say "calling." With texts, the sending bar gets to about 95% then just stops. I have tried texting/calling multiple people with different phones/carriers. I have taken out the sim card, and turned it on airplane mode for a while. There's really not much information on the internet, as this seems to be a pretty rare problem. Any advice would be greatly appreciated. Thanks.