on 12-09-2019 20:30
Check for faults here https://status.o2.co.uk/
if nothing reported, your sim may have failed so pop into your local o2 store for a check and free replacement.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)
on 12-09-2019 20:31
Check your postcode here:
Is your account up to date?
Clean the sim card and then reboot again