Don’t forget about gravity… it’s always here! The accelerometer is giving you the vector in the direction of gravity, the difference between how much the phone is accelerating and how much gravity wants to accelerate the sensor.
If you hold the phone over a soft surface and drop it then momentarily the acceleration readout will be very close to zero, because both the phone and its accelerometer are feeling (about) the same acceleration, gravity.
I’d start with the usual googling for this. Might be your phone, might be an OS-level setting, might be a setting in PlayerPrefs (Unity), might just not be supported on your phone, etc.
Maybe you just need to reboot the phone because the Android daemon in charge of the gyro died. Android is flaky like that.