No Sim

you can also turn the phone off..remove the sim card..wipe it down really well..put it back in and turn the phone back on in case it was either dirty or moved around in there a bit..not unheard of for a sim to go out on you however its not a frequent thing to happen either. so could be the sim card itself.

I googled. Try shut down/restart. Be aware it could be a problem with the network, not just your phone.

I am having the same issue, but it is now happening 5+ times a day. Restarting works but only for so long before it does it again. It's a deeper issue than just restarting the phone