Hi SAN5, welcome to the community!
That is strange, have you tried simply restarting the phone and removing and re-inserting the sim card? If you have and it still isn't recognized, try a different sim card in the handset to see if that can be recognized. If another sim card is able to be recognized, you may just need a new sim card. If no sim cards can be recognized then it may be an issue with the sim card reader, in that case I would advise contacting your local Sony Xperia Customer Care team.
