Please check your area coverage from here: http://status.o2.co.uk/ Just by entering the area post code. If everything is Okay, then reboot your handset just by tuning it off and on. Hopefully, it will work.
If you're still facing same issue, try your SIM in other unlocked handset and vice versa. This will help us determine whether the fault is with the handset or the SIM. If the SIM works in another handset then it's a handset issue which you will need to address. If the SIM doesn't work in any handset that means the SIM is faulty and must be replaced by doing a sim swap.