Do you mean you can't get 4G signal? Did it happen to your other phones (if available)? I'm not sure its also your network provider's issue. Or maybe your phone needs a software update.
Try first to reboot your phone and test. Next turn your phone off then remove the sim card(s). Turn the phone on without the sim let it identify that there's no sim inserted. Then turn the phone off and re-insert the sim(s). Now try to see if you will get 4G signal.
If still no go, you may try to set your network signal to auto or manual to 4G.
Last resort is to follow the instructions in this website article to "reset" your network settings back to it's default.