LD150 wrote:
Most likely it has run out of battery or the "finder" has taken out the SIM
Or it has no Internet connection with which to report its location – e.g., cellular data was turned off, and it's not near a Wi-Fi network it's previously joined. Or the thief has put it into a special kind of bag – one which blocks reception of radio signals – to keep it from connecting to the Internet.
If the OP had Find My Network turned on, there's a small chance that the phone could signal its location (with a little help from nearby Apple devices) even if Wi-Fi and cell data were turned off.
Do not respond to any texts offering to return the phone as it will be a scam .
The senders may pretend to be from "Apple Support" or "Apple Security"; but it's all a lie & a scam.
These texts will be from criminals who want the OP to clear the Activation Lock, or give them the information to clear the Activation Lock (so they "can make the OP's phone their own."). Or who want the OP's credentials, to scam the OP in other ways (identity fraud). Or both.