Same thing has happened to my M11 and a friend's M11. It can happen if it falls onto the wheel or sometimes while packing it up or just during travel.

You can do this without taking the wheel off just by pulling on it, but there's a chance you'll break the wheel....

All you need to do to fix it is take off the wheel, grab the plastic piece that the wheel attaches to with a pair of pliers and pull it until it makes a little pop (it takes a little effort). Put the wheel back on and it will be as good as new.