I have hella spotlights, but they should all be the same design: my S2 ones are same.

Look at the bottom of the spotlight in the pic below. The small black rectangle is a plastic bracket which screws into the bottom of the black plastic outer-housing. (This is normally still attached to the housing in one piece and hinged via a plastic-weld, but if the light has been out a few times they can become separated). The top of the light stays in the back cover via a nodule which sticks out from the rear of the glass and into a small bracket which is attached to the rear plastic cover. If the lower bracket has come away, there is nothing to hold the lens in.