You didn't say whether the wheels came off the axle (both at the same time highly unlikely), or the axle came off the fan case (more likely)?
As you probably know, axle is held to the fan case with two screws, and two little "keepers" (for the lack of better word). VERY simple to reattach (two screws!). You have to insert the spring into the height adjustment gizmo, and align this FIRST, and then place the axle into it's grove on the bottom of the fan case, and screw in two little keepers. That's it. Hardly difficult, and definitely self explanatory once you flip the motor over.
