Sim not recognised after update

Sounds like your phone was jailbroken to unlock if from the earlier carrier and when you updated the ios it relocked to that carrier. To be able to use the phone you need to have a sim from the carrier to which it seems to be locked. The other alternative if your phone was not jailbroken is that you have a faulty sim and need to get a new one from your carrier

Most likely, the phone was previously hacked to do an illegal carrier unlock When you updated, the hack was removed and the phone re-locked to its original carrier. It will now recognize only their SIM card and operate only on their network.

Hacking to unlock the phone doesn't require jailbreaking. Most of these phones aren't jailbroken in the usual sense. The vast majority of phones sold on ebay have been hacked in this way. They work "perfectly" untile they are iOS updated or restored... t

If you got it new from O2, it wasn't hacked. Replacing the screen shouldn't cause this, but there could be other damage. Try replacing the SIM card.