The same thing happened to me on my 24-105mm. One day, I took the camera along with the 24-105 out of the camera bag and noticed something wasn't right. I looked at it... I grabbed it. The red ring just came off. It broken into three pieces. I was like "What???". It was like a dream.
Long story short, since it broken into three pieces, I couldn't glue it back on. I decided to put it up for sale. To me, it just didn't look right without the red ring.