No sim no network why?

[Miko]

I have information there is no sim no network? Why? Yesterday was everything ok,?

[TallTrees]

Look here @Miko 

Guide: How can I sort out my network issues? 

Guide: Is the network down for me or everyone? 

[Oxonian]

This is another resource @Miko that is worth a look :-

My Network App - Find Out More About Your Network Coverage | Apps | O2

https://www.o2.co.uk/apps/my-network

[pgn]

Check your SIM is correctly seated in the handset - sometimes removing the SIM, if it is a physical SIM, and cleaning it gently with a dry, lint-free cloth can help. If you need a replacement SIM, take photo ID with you to an O2 shop, as standard procedure these days, when you request a replacement SIM online, is to bar/deactivate the current SIM, leaving you with a wait of several days while Royal Mail delivers your new SIM card, @Miko - good luck!  https://www.o2.co.uk/help/device-and-sim-support/help-with-mobile-broadband/problems-connecting