Is your wheel not rolling properly? Did your wheel come out completely?
If you pull the liner away and look down toward the wheels, you should see a plastic tab insert (probably on the "good" side, if one side isn't working properly - can also see thumbnail images provided). This tab is what holds the axle in place. If the tab comes out, the axle can either partially, or fully, come out of place, which will cause problems. The wheel may not roll smoothly, or it could fall out of the casing altogether.
1. Casing and wheel are still in good condition, but the wheel fell out - you can purchase this set to replace the tab.
2. If your casing is cracked in any way, or your wheel is broken, you can purchase a Wheel Replacement Set