Activation temporaily unavailable ios 6, iphone 3gs

I have an iPhone 3GS, with a Bell Sim card. I updated to ios6, but when I try and activate it, it comes up with a message saying Activation is temporaily unavailable. Also, when plugged in to iTunes, It says there is no sim card installed in the phone, but there is one. Anyone know how to fix these problems and get my phone working?