Hi.
Do you see a 4g,3G , e or 0 symbol in your phones top status bar usually next to the o2 network logo.
Make sure 3G and mobile data is turned on in your phone.
Try Settings , general , reset , reset network settings.
Check your service status for mast issues.
http://status.o2.co.ukGive your sim a clean as this can help.
Also check your apn settings in your phone.
http://www.geeksquad.co.uk/articles/chapter/595-04-how-to-set-apn-settings-for-the-02-mobile-networkIf they are ok then ring customer services in 202 contract or 4445 payg to check your account.