It is an easy fix.
It happens all the time when someone bumps into the front or the backside of the Twizy. A small force is enough to tear the attaching lips off. Point is nearly no Twizydriver knows his or her lips have been broken off too
The bumper as you call it has two lips at the bottom. Those lips are bolted to the chassis with 10mm bolts.
Around the lips the bumper pivots from horizontal to vertical where it clicks onto the upper side of the front fascia.
So this is what you do.
Remove the broken lips by loosening the 10mm bolts.
Drill two holes next to the bolt hole in the lips, on each side of the original hole one hole. Use a 5mm drill.
Drill two holes in the bumper where the lips are treared off. The same 5mm drill.
You place the lips on the tear off spot of the bumper and drill 4 holes where they correspond exactly with the freshly drilled holes of the lips.
Now pull 4 tywraps through the corresponding holes. Be aware that you pull them just enough as if the lips were attached to the bumper.
Bolt the lips of the bumper to the chassis and click the upperside into the fascia.
Be assured you don't see the tywraps when the bumper is on the Twizy.