Bumpers can come off during an accident or just by bumping into a curb while parking. The part of the bumper that may fall off is called the bumper fascia -- the outer plate covering the actual bumper. The fascia provides extra spring upon impact, protecting your radiator and other vital engine parts from damage. Hopefully, a few pins will have popped loose and can be reinserted for an easy fix. If not, the pins have probably broken and need to be replaced. Push-in retainer pins can be found at most auto body supply stores.
Ensure that you park the car on level ground with the wheels facing forward. Allow plenty of clearance on all sides of the vehicle. Engage the parking brake to avoid risk of injury during installation.
Place the fascia back on the bumper, centring it on the car. Help from a friend may be required to keep the bumper fascia in place while the next step is completed.
Insert push-in retainers (also known as "push pins") into the fascia. On the front of most cars, there are retainer holes located on the top and bottom of the fascia as well as inside the wheel wells. A rubber mallet may be needed to help insert the push-in retainers.
Reinstall any loose light covers with a screwdriver, and reinstall the grille if needed. Most grilles can be screwed in, clipped on or both. Test the lighting system before screwing in the covers to save time.
Some models may require clips to latch or screws to be inserted into the fascia as well. Refer to the owner's manual for a diagram of the specific model's bumper.
If the actual metal bumper is damaged, it will need to be straightened with a frame machine to correct the problem. Speak with a mechanic to have the crossbar repaired correctly.