First, try rebooting your phone. If you still have issues then check if there is network maintenance going on by going to https://status.o2.co.uk/
My mobile data has been working fine for most of the day up until about 4pm now no 4g etc since, still have 2.5gb left on my goody bag.
I suggest you check the APN/Data settings again on your device to see if they are correctly configured. See this
That does sound odd.
Check Wi-Fi is switched off and that mobile data is switched on.
Also check the right APN is selected as it might have defaulted to another preset.
Also check preferred network type is 2G/3G/4G Auto