Contact Sensor Disappeared

I have ring alram setup for more than 1 year and all of sudden one of my door sensor disappeared from it. It was on back door and was set to trigger without any delay, someone opened the door and alram didn't trigger (it was in home mode and suppose to trigger alarm). I double checked and contact sensor is there on the door and missing in the app. It is security hazard. I don't know when it disappeared and what is wrong with it, there was no warning or indication and I am shocked that this would have created a threat during emergency. I tried manually and sensor doesn't even chirp anymore. I can't turst this system anymore. Any pointers? Thanks.