Uninstall Jailbreak iPhone 3G

on your iPhone choose settings->generel->reset

I would try that.

See if you can force the phone into recovery mode & restore as new:

Turn your phone off. Leave the USB cable connected to your computer, but NOT your phone, iTunes running, press & hold the home button while connecting the USB cable to your dock connector, continue holding the home button until you see “Connect to iTunes” on the screen. You may now release the home button. iTunes should now display that it has detected your phone in recovery mode, if not quit and reopen iTunes. If you still don’t see the recovery message repeat these steps again. iTunes will give you the option to restore from a backup or set up as new. Select "New".

Hello wjosten

Thank you so much for your reply: I appreciate! Does Selecting "New" mean I will loose my contact data and so forth that I have already synchronised with iTunes?

Cheerz!

on your iPhone choose settings->generel->reset

I would try that.

Yep. However, your contacts should be in a supported contact application on your computer. If not, you will lose this data.

I can't guarantee that this will work. Fact is, depending on how your phone was jailbroken, you may never be able to install a stock iOS back on the phone. If it doesn't work, your phone will be a brick & we will be unable to provide further support on this forum. One of the downsides to jailbreaking an iPhone.