Hi
If you have upgraded and are now using a nano sized SIM card you need to perform the sim swap here
http://swapmysim.o2.co.ukIt takes from a few minutes to approx 24 hours to settle.
This moves your tariff and mobile number from your old sim to the new one. The service will stop on your old sim first indicating the swall has started
If you have performed the sim swap then turn airplane mode on and off every 30 minutes or so to help with the re connection.
Also try Settings , general , reset , reset network settings.
Take a look here at your masts status too
http://status.o2.co.ukRemove and clean the SIM card.