stolen or insurance claim and its been reported lost
if bought genuinly then contact the person who sold it to you and get them to contact their phone company and remove the block on the phone

the phone is blocked, not the sim and it wont work on any uk network

if bought from someone you dont know then good luck, the proper thing to do is go to the police and get them to check if it is stolen or not.
if not then contact your phone company and ask them why its blocked, you may get an answer you werent expecting, like someone has reported it lost.