Since you're getting no service on your phone -
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.
You can order for a SIM from here: https://giffgaff.com/orders/mgm
This can take up to 2-3 working days but it shouldn't take that long.
Swap it here: http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
Once you do a SIM swap all the account details of your old SIM will be automatically transferred on the new SIM along with your goodybag and it take up to 24 hours.