Contact O2 from either a landline or live chat
http://www.o2.co.uk/contactusYou will likely have a block on your account which will need to be lifted and the replacement sim activating.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.