I had the same problem as the OP come up a little over a month ago after switching sim cards. I went from tmobile to platinum tel which is a tmobile virtual network operator so the apn actually stayed the same. There were no other changes to my phone. The only way I got the phone back to normal was to do a hard reset (soft reset didn't work). I suspect there is a config file with the info somewhere in the user's home directory, but not sure where it would be.