SCENE 1

 So your trashcan's tire came off the wheel. If you've tried to wrestle it back on and only accomplished a few new curse words, welcome to the club. At this point your only hope is to remove the wheel from the trashcan.

SCENE 2 
You may want to look at scene 4 and come back.

 Near the center of the wheel is a little secret hole exposing part of a spring/plunger assembly that is holding the wheel on the axle of the trashcan.
  Spray some light lubricant such as WD-40 to loosen up the rust.
 If this is your lucky day, you could theoretically use a small tool to pry the assembly towards the outside of the wheel and pull the wheel off the axle and skip the next scene. Or you could proceed to the next scene.

SCENE 3 

 On the rim of the wheel is a cap that keeps tension on the spring/plunger assembly. Pry off this cap and now there's no tension on the spring. You should be able to pry the spring/plunger assembly away from the axle as mentioned above and remove the wheel.

SCENE 4

 Now that it's half over, let's gain some understanding of how the wheel is held on. The spring/plunger assembly is shown twice here (but there's only one).

Circle 1- This shows how the assembly slips into the wheel. The white cap frome scene 3 keeps tension on the spring.

Circle 2- This shows how the plunger snaps into the groove of the axle to hold the wheel on.

SCENE 5

 Now that the wheel's off you can have your way with it.

BEFORE YOU PROCEED BE SURE TO REINSTALL THE SPRING/PLUNGER ASSEMBLY AND WHITE PLASTIC CAP!!

Use a flat prybar to work your way around the wheel and pry the tire back on. A little at a time. You'll probably bend up the lip of the plastic wheel, but hey! It's a freakin' trashcan! If you pry from the inner side of the wheel you won't see the the bent part anyhow.

Once the tire's on you can simply push the wheel onto the axle and lock it back in place.

WHALA!

Ready to Roll !