Good Morning,
Remove your sim card and give it a wipe to remove any static build up.
Check here to make sure their are no issues with the masts near you
http://status.o2.co.uk/
Check with 02 that your account is live and activated by dialling 202 for contract customers or 4445 for payg customers.
Check their is the 3g logo in the top status bar or the circle or e for edge to show Internet is on.
If not check in your settings to ensure 3g is toggled to the on position.
Check your APN settings to ensure they are correctly entered.
Here is a handy link for these settings.
http://www.geeksquad.co.uk/articles/chapter/595-04-how-to-set-apn-settings-for-the-02-mobile-network
