Re: Headlight cluster
Is the piece you are talking about a circular pressed metal fitting with the securing spring attached to it? If it is then these are usually fitted using a press during the manufacture of the lamp and are an interference fit. If it has come loose then you will probably find that it has also damaged the bits of the reflector upon which it interferes. It is highly unlikely that you will ever get it to fit properly again. You may be lucky though.
You will find that it will probably only fit in one orientation, try to align it up correctly and push it back onto the reflector, ensuring it goes all the way to the stop. I believe it is an H7 bulb, then the notch that takes the tab on the bulb will be at the top. Be carefull not to apply too much pressure to the reflector as it is quite easy to push them off the adjusters...if this happens then you'll need a new headlamp.
I very much doubt that a dealer will contemplate doing this sort of remedial work and will more than likely just swop the lamp out for a new one. An independant specialist might give it a go. If they will then their best bet is to remove the lamp from the vehicle and work on it on a bench, they'll never do it on the car.
Hope this helps, however you may find that you can't get the ring to stay in place and that will mean a new lamp...sorry.
Oh, and do not on any account think of using any type of cyanoacrylate glue to hold it in place. When these, and many other types of adhesive get hot, they give off gasses that will coat the reflector and lens and then you really will need a new lamp. The only thing that we find can work is an epoxy.