When your SIM card isn't detected, your phone won't make or receive calls. Your screen freezing and SIM card trouble may be connected, but they could also be two separate problems. Try to solve each problem to see whether or not that solution also ends the other problem.
Reset the Phone
The first step when troubleshooting any problem with a smartphone should be to turn the phone off and then back on. After powering down your phone, remove the battery, set it aside and wait for a minute. Next, put the battery back in and turn on the phone again. If it detects the SIM, then the problem may be solved. Apple also recommends switching Airplane Mode on and off if the SIM isn't detected.
Check your phone for physical defects that may be preventing it from detecting the SIM card. Remove the SIM card from the phone and examine it to see if the card is warped or otherwise damaged. Check to make sure the SIM slot is the right shape and that it hasn't been damaged. The contacts on the SIM must make contact with the contacts on the phone; a defect on either the SIM or the SIM slot in the phone can prevent that connection from occurring.
If the SIM card and phone are completely normal, the problem may be with your software. If your phone has a safe mode, start it in safe mode so that only apps that came with the phone load. If not, uninstall any recently installed apps and check to see whether the phone stops freezing. If removing third-party apps or safe mode stops the problem, then one of the apps that you installed before the problem started may have been at fault. Also check to make sure that your OS is up to date. You may want to reset the software on your phone completely to see if that fixes the problem.
It's possible that a sensor that affects your screen is being affected by a case. For example, Sony's Mix Walkman phone has sensors at the top that detect proximity and can turn off the touchscreen. Your phone case could be touching part of the screen and affecting how it works or even overloading it with too many interactions and causing it to freeze. Try removing the phone from a case and not using accessories to see whether they're the cause of the problems.
You may be able to live with a phone that freezes occasionally, but your phone won't function if the SIM card isn't detected. The SIM card is what activates your phone on the mobile carrier's network. Take your phone to a carrier store and explain that you can't connect to the network because the phone won't detect the SIM, even though there are no physical defects. The problem may be with the SIM and your carrier can replace it.
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