My sim card has been stolen from my phone

I left my phone in a shop today by accident but was only gone 10 minutes. It was there when I returned but some time later, I realised I had no call data and it said there was no SIM in the phone - I’ve since had a look and the SIM has gone. I’m trying to call NOS or find a chat somewhere to get my SIM blocked but not getting anywhere!!! Please assist urgently!!