I had this problem today. It had nothing to do with any of the posted answers. I tried them all. My problem was a misconfiguration with the zaptel.conf (running old asterisk!). I had commented out some span details trying to troubleshoot a PRI issue. When the server got rebooted this issue presented itself. I was able to discover this by looking at the wanrouter messages in the logs. I noticed there were 3 errors, and the corresponding line numbers, once I corrected these issues, and ran an #asterisk -vvvvc the server came back up and everything worked.
