Could be a service outage, have you checked https://status.o2.co.uk ?
Are all your bills upto date?
We’re just customers like you but it sounds like you’ll need someone to access your account which can’t be done from here. If you message O2 on Facebook (https://o2uk.co/O2CFB) , Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG) , they should be able to help you with this
If 4G has been switched off, why don't you try 3G in your network settings? They may be upgrading a mast near you.
Check this guide Guide: How can I sort out my network issues?
If your phone no longer works at all, then I wonder if you have paid your bill?
Either way we have no access to your account, so contact O2 via social media
*The Game Is On*
Just try rebooting your phone.