The toggle pedal you push to engage/disengage the wheels fell off

The pedal assembly must be replaced,( part # 558401S), as it has a gear pressed to it which has come loose. The handle, switch plate, and upper motor cover must be removed first. To remove the plastic switch plate, remove it's screws and insert a flat screwdriver into the rectangular slot in it to pop it off. Four screws hold the upper motor shell in place. Next the switch button must be removed. The Techdrive assembly can be removed by removing the three screws on the bottom. Drop the drive down, and unhook it's belt from the gear on the end of the armature shaft. The pedal assembly can be removed by removing the remaining screw from the bottom of the drive unit. The N/D pedal has two parts - the pedal itself, and the shift cam. Observe carefully how these units fit together. After installing the new pedal and replacing the screw and washer to secure it, re-install the drive unit. The shift must be in the Drive position to install it, and remember to replace the belt. Be sure to line up the arm on the drive unit so that it engages the slot at the base of the handle pivot and re-install the other items.

First remove the bottom plate, brushroll, and belts. Three screws hold the top cover in place. One screw is in the center just behind the brush. The others are midway up on the sides - you may need a flashlight to see them. Once the screws are out, there are two latches on either side that secure the top cover, and these must be pushed outward with a flat screwdriver. Push one out to loosen the cover, and insert a screwdiver between the cover and the lower body to secure it while you push out the other side. You may want to remove the lower hose as well to make more room while replacing the drive actuating arm.

This requires removal of the transmission assembly and replacement of the pedal assembly - it cannot be repaired. The handle and upper motor shell must be remove to access the transmission assemby. There are two screws in front, and two under the plastic switch plate.

The pedal assembly must be replaced, as it has a gear pressed to it which has come loose. The handle, switch plate, and upper motor cover must be removed first. To remove the plastic switch plate, remove it's screws and insert a flat screwdriver into the rectangular slot in it to pop it of. Four screws hold the motor cover in place. Next the switch button must be removed. The Techdrive assembly can be removed by removing the three screws on the bottom. Drop the drive down, and unhook it's belt from the gear on the end of the armature shaft. The pedal assembly can be removed by removing the remaining screw from the bottom of the drive unit. The N/D pedal has two parts - the pedal itself, and the shift cam. Observe carefully how these units fit together. After installing the new pedal and replacing the screw to secure it, re-install the drive unit. The shift must be in the Drive position to install it, and replace the belt. Be sure to line up the arm on the drive unit so that it engages the slot at the base of the handle pivot and re-install the other items.

The spring goes inside that pedal that you step on to advance the height. Place that spring in that pedal and then place the pedal into its' groove, then put the axile and wheel assembly over the pedal and put your screws back into the mounting sections.

you need to make sure the belt is on the roller correcty, make sure is at the red position when putting the part back on with the belt on the hook, push the pedal to highest (off the ground) position to ensure you put it on correctly, then turn the fixing clip thing to secure it, then once its on turn the panel around to green position, belt will be back on, once you get used to it you'll be ok, if you know this already I apologise in advance :o)

There is a small groove in the end of the stud on the vacuum that the wheel fell off of.
Slide the wheel back on the stud, put the washer on then put the ends of the half clip in the grove on the stud pushing on the back of the clip with a small screwdriver on the clip to lock it back in the groove. The old parts may be still in the wheel if it rattles. There should be a plastic cap on the wheel like a little hubcap, pry it off with a screwdriver carefully so you can put it back on after the clip.
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In Kirby vacuums with tech drive there is a part called Bracket cam (see it here) that is designed to disengage wheels from transmission. When it starts wearing out Kirby would make crackling noise when on neutral and eventually may not disengage at all. If there was no stage where it was noisy on neutral it may be something else - most likely the pedal assembly broken (see it here). It is not very hard to replace it.

you need either a new nutreal cam bracket assembly, that is what engages and disengages the tech drive.OR to adjust the handle pivot assy- diffucult . this is not for the faint of heart. it is possible to fix them as i used to repair kirbys, however, I remember how frustrating it was to figure out how to disassemble and reassemble them. it is very complicated to a novice. I would recomend having it repaired professionaly. it is possible for a professional to repair it in a very short period of time.

another thing to consider. look on the UNDERSIDE of your Kirby, between the two drive wheels. just in fronnt of the tech drive/nutreal switch. there should be a screw there, tighten that screw up. actually tighten all of them. on My generation 3 the center screw came loose and it would not go in to drive and it would make an aweful clatter. hope this was helpful