my phone is telling me there is no sim card installed when there is?

Can anyone help?

I have had to remove my sim card as i was told to do this by o2 as my 3g was not working, i have now put it back in and it is telling me there is no sim card installed!

i have made sure it is clean and in the correct way and it is still not working, i have also refreshed and restarted the phone and my network settings but nothing is working?

Can you help me?