We're going to need more info from you please.
Do you have any data allowance left http://www.o2.co.uk/myo2 ?
Is there a local network issue O2 Service Status ?
What phone and contract do you have?
Is it an issue everywhere or just in one location?
Have you tried resetting your network settings, rebooting phone etc?
Have you tried all network modes, 2g, 3g, 4g etc?
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