You need to remove the shade and take it apart. There is a small black plastic wheel that has three raised dimples that serves as the position stop. The dimples are worn down this shade drops all the way down.
You can order a replacement part from MCD or fix it simple by using something hot... like small soldering iron and touch it at those worn off dimples. This will melt and raise edge of plastic forming new stops.
See attached pic and look at circled black wheel.. this is what has worn down. Towing with shades fullly down outs stress on this stop wheel and it will fail again ... tow with shades fully rolled up and you will never have another issue
This link video shows how to take MCD roll up shade apart and reassemble.