Ok, SIM card in and out has rebooted the phone so that's all covered.
Could the phone be blocked (reported lost/missing etc) or have you had it from new ?
Get the SIM card and put it face down on apiece of clean white paper, just move it around in circles pushing on it for 10-15 secs. You're cleaning the corrosion off the chip, it forms between the contacts on the phone and the sim over time, normally it's fine but taking it in/out or it moving in the holder slightly can make it lose connection (increased resistance). The paper should show a slight black mark on it when you've done, this is normal.
Type *#06# and make sure the IMEI hasn't crashed and changed to 12345 etc or all zeros.