Drive shaft spinning and it will not go into gear.

the drive shaft just drives a pinion gear in the prop box that drives to gears that spin in opposite directions all the time
by selecting forward with the selector rod , a dog gear is moved forward to mesh with one gear that is turning or moves the other way to engage the other gear that is turning
that is the forward or reverse action for the prop
if you have forward but no reverse the it is not a drive shaft problem but a selector rod adjustment that is not moving the dog far enough to engage the running gear

check for a roll pin in the gear or shaft that has sheared off
check that the square end of the drive shaft is not rounded off or that the square drive in the gear is in fact locking on the drive shaft and not turning from worn out

Pull the final drive apart and take a look at what broke because something did.

To give the mixer strength it has planetary gears. Something may be wrong in the gear section

sounds like you have a busted drive shaft or a bust spigot shaft going into your gear box. Easiest way to check on this is to jack up the front of the vehicle and spin the wheels. If the drive shafts move then they are fine and you have a busted spigot shaft. The only way to replace this shaft is to drop the gear box and replace it.

wrong!!! if the bottom set of blades aren't turning its because the drive gear for that shaft is loose and is spinning on the shaft instead of spinning the shaft itself. try tightening the screws but make sure the screws are lined up with the holes in the shaft so they seat otherwise your going to strip the plastic gears threads. if that doesnt work then you will have to replace the gear and shaft. its a cheap part and readily available. only problem is youll have to take apart almost the entire heli to get the old one out n the new one in. the gear comes pre-mounted on the shaft. Buy 2 or 3 of them right away as you will have to replace it just about every time your heli tips over while the blades are turning. the screws only go into the shaft about a 16th of an inch and is easily stripped out

I believe you can see the actual shaft for the shaft drive spin. If you start the motor and put it in gear and see the shaft spinning the problem is a stripped pair of gears in the final drive. Compare the cost of the pieces (plus labor) to the entire assembly, the assembly will probably be cheaper. If you can not see the shaft just check the oil in the final drive for metal chips. A worst case scenario , your engine case has metal chips ( That might cost more.).

If you are putting the bike in gear while it is running, and the drive shaft is spinning but the cycle is not moving forward, then you have a broken or stripped gear in the final drive. Try draining the hypoid gear oil and open the fill cap to inspect for damage and metal chips.Your best option is going to be to replace the final drive assembly. You may be able to purchase all the parts to rebuild the old one , but all the bearings and gears may cost as much as the whole assembly.A good option may be to look for a good used one.