Look at the left (I think) side black lower cover (where a fog light is or would be) for a hole about maybe a half inch diameter.
The sensor is in that hole.
Generally, if you have NO readout, the wire got knocked off the back of the sensor.....you have to remove the plastic horizontal cover to see the backside, bottom of bumper cover......just remove enough fasteners so you can pull the panel down enough to see/reach in there.
The car may have had the front bumper cover removed for service or repair and the harness wasn't fully plugged in.
Rob