Q: why does my iphone say no sim card detected

I have tried reseating it many times to no effect. Took phone to my operators shop ( O2) who said nothing wrong with SIM Card, try restore on Itunes and if that doesn't work go to Apple shop 20 miles away!

Cant find restore on new version of Itunes, but have not reset all settings on phone and restore from backup, but still says no SIM card.

By the way its an Iphone 4S

Peter, thanks, I have already treid a lot of this. The bit I cant make much sense of says

Restoring your iOS device

1. Connect your device to your computer.
2. Select your iPhone, iPad, or iPod touch when it appears in iTunes. Select the Summary tab, and click the Restore button.

Number 2 here doesn;t seem to be available in my current version of Itunes. I no longer see the column on the left which enabled me to "select" my Iphone. I can find it under "file" but there is no summary tab, and no restore button.???

Thanks, it was the View/Show sidebar part I was missing. Restore now in progress, will update on progress.

Peter, many thanks for your help, I have completed the restore now, or at least part completed it! All went went well till we got to the stage where it sais something to the effect of - No SIM card installed, please insert tomorrow. Looks like a visit to the Apple store tomorrow to see if they will exchange it - again!