I'm sure you will have already tried turning it off & on again? This should make it search for a network. If the battery is removable you could try taking it out whilst the phone is turned on, then replace it & turn the phone back on, This will reboot the phone. (If the battery isn't removable, try holding the power button down & it may give the option to reboot).
Are you able to go into the phone settings & manually select a network?
If non of the above work, you could also try taking the SIM card out & putting it back again, then turn on the phone.
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