I purchased an LED headtorch that has an adjustable lens. When the lens was pulled out to focus it, a plastic ring would dislodge and block the LED. The fix was simple: glue the plastic ring in place with a hot glue gun.
The lens unscrews off, in the middle is the plastic ring. I'm not sure what the ring does, but I think it helps direct the light from the LED, and/or keeps the LED board pressed down at the right angle.