Hi monica81
It is possible the SIM card is faulty.
Can you try your SIM card in another phone as a process of elimination.
If it is the sim then any O2 shop can transfer your tariff and mobile number to a new SIM card. Takes from a few minutes to approximately 24 hours to settle.
The service on your original sim stops first which indicates the swap has started ,and then turn your phone off and on approx every 30 minutes until full service begins.
Or have a sim posted or pick one up from the O2 shop and perform the sim swap yourself via
http://swapmysim.o2.co.ukBe sure to ask for the specific account type blank SIM card. Ie Contract or Payg as they hold different info on them.