Using simple ways to eliminate the problem, you can return to the network quite quickly and without serious problems.
When a smartphone has network problems, it can happen suddenly, even if everything was working fine before. The problem could be due to a software crash, SIM card issues, or hardware malfunctions.
Simple crashes can usually be fixed by restarting the device. To do this, hold down the lock button until the shutdown and restart buttons appear, then select Restart. If there is no Reset button, turn off the device and turn it on for a few minutes.
Another way to reset the wireless communication modules is to turn on airplane mode for a short time and then turn it off. This will help restart all modules at once. If you have at least one of the modules still working (for example, Wi-Fi works, but no cellular connection), you can try turning Wi-Fi off and on first to check the connection.
If rebooting and turning on airplane mode didn't help, you can try resetting the network settings. In the smartphone settings, find the “About phone” or “System” section and select “Reset network”. This step will help to return the network settings to default values.
If all these methods did not bring any effect, the only thing left is to replace the SIM card. Update it with your operator, issue a new one, while you can keep your number, it will take a few minutes.