First thing, dumb but it's a first, check if your NFC is really turned on. If it's on, restart the phone.
If that didn't solve your problem, test if your NFC is working; search for "NFC test" in Google Play Store and test with a NFC tag or card or other smartphone (the app will show the ID and other info, regarding the NFC connection). If the NFC is turned on and nothing happens, you may have a problem (maybe logical - a bad app/system cache or maybe a physical one - on the coil or something like that).
If that confirms, you can try first reset the Google Pay app (clear all data/cache from it) and reconfigure it. If that didn't work, reset the whole phone. If that last resort didn't work too, so you may have a bigger problem and probably should take your phone to the nearest care center.
Wish you luck.