The parking sensor is held by 2 clips on top and bottom which are part of the larger plastic trim ring that is pressed into the bumper. So, you have your bumper with the 4 holes in it, then a plastic trim piece/ring is pressed into the bumper, then your sensor pushes in and snaps in place from behind the bumper with the second piece. Maybe it just unsnapped or a tab could have broke on the trim piece? Have to reach behind the sensor and push it into the trim piece, if it snaps in your good, if not, see what's broke? ( there is also an o-ring that sits around the sensor to keep it snug in the trim piece, make sure it's not keeping the sensor from popping in place) You do not have to remove the bumper for any of this.