Trash cans with wheels make it much more convenient for people to bring their trash out on collection day. Trash cans can weigh a lot when full, and people with bad backs and other ailments may not be able to lift or drag them without risking injury. One problem with wheeled cans is that the wheels can fall off. If the damage is from a broken plastic bracket on the can itself, the can will likely need replacement. However, if the metal bar that holds the wheels on has warped, you can put the wheels back on with minimal effort.
Examine the bottom of the can to determine why the wheels fell off. If one wheel is on and the other is off, pull the bar out on the side where the wheel is still on. It should slide out with little fuss. Remove the other wheel by pulling it off. If you can't get it, stand on the wheel and twist the bar as you pull up. Once the wheel is off, you can better examine the bar.
Roll the cross bar on the ground to check for bends. If the bar is warped, it will not roll smoothly, and you will see a dip in the bar as you try to move it. The bar could have been warped from a car hitting it or from someone pulling the can up a curb at high velocity, forcing a wheel off in the process.
Hit the bent part of the bar with a rubber mallet to straighten it back out. Use minimal force so you straighten the bar without crushing or damaging it. The bar must remain round overall for the wheels to roll efficiently.
Place the first wheel back on the bar. You may need to hammer the opposite end of the bar with the mallet to force it back into wheel.
Slide the bar back into the bracket at the bottom of the can and turn the can on its side, so the wheel that is on is facing the ground.
Position the remaining wheel back onto the bar and hammer it in the center with the mallet to force it back on the bar.