A-9 Grinder Diagram

A bench grinder has left handed threads on one shaft and right handed on the other,so that the grinding stones don't unscrew the nut as you grind something.I haven't ever gotten around to getting a bench grinder,but on my father's, I used to just hand tighten the nut,then grab the grinding wheel with my hand while tightening the nut with a wrench.(You don't have to make it real tight,as the wheel pushing against the nut will keep it tight)

If there isn't an arbor nut holding the blade/disc on the grinder you have a disc with the threaded center built into it. While holding the arbor lock button down you have to turn the disc by hand until it breaks free and spins off. It may be on vary tight as it is a self-tightening thread so the disc won't come off while you use it.

sounds like the shear pin is broken

sounds like a broken shear pin

I am assuming you refer to the spinning of the motor, but not the grinding mechanism? Often times, the blade, grinder, or whatever does the actual work is on a shaft from the motor. There is a 'shear pin' through the blade, &tc which not only holds it on the shaft, but causes it to turn. If something jams the cutting part, the pin 'shears' off, to protect the motor from burning out. It sounds like that is what has happened to your grinder. The shear pin bits must be removed from the grinder and the motor shaft and a new one inserted. Usually, a common 'roll pin' will make a suitable substitute. Just make sure it is the proper diameter for a tight fit.
The motor shaft spins, but the blade, &tc does not turn.

the shear pin, which goes through the grinder blade base and the motor shaft has 'sheared' off the protect the motor from overload. The metal bits of the pin need to be removed from both the grinder part and the shaft, a new one inserted. It is a 'pressed fit', so it will be hard to remove.
After the bits are removed, a common 'roll pin' of the proper diameter should be a suitable replacement

There is a nut that you release from the middle of the grinder wheel, (grinder blade)
This nut is flat and has four holes in it.

You will see you are providing a spanner wrench to remove this nut. The wrenches' actual name is Lock Nut Wrench. It has a U shaped end with two pins sticking out. On on each side of the U shape.

The pins are inserted into the holes in the flat lock nut, located in the middle of the grinding wheel. The wrench is laid flat against the grinding wheel, with the two pins of the wrench going into any two holes of the nut.

At the top of the Makita grinder you will see a Black button. The button is pushed down to keep the grinding wheel from spinning. Holds the grinding wheel in place, as you turn the lock nut loose. When you release the button, a spring inside pushes the button back up out of the way.

As you view the Makita grinder, Release Button towards the Back, grinding wheel towards you, the nut is turned to the LEFT, to loosen. It is a Left threaded nut.

The grinding wheel spins to the Right. Using a left threaded nut insures that the nut will stay tight, due to the direction the grinding wheel spins.

I was able to find 3/32 x 1/2 inch spring pins at a local hardware for 13 cents each. They may not last as long, but at the price I can replace as needed.

I also had to disassemble twice; I didn't notice that a portion of the old pin was wedged in the underside of the burr. When I put it back together the first time, the new pin rested on the old fragment and pressed the burrs too close together. On the coarsest possible setting, I got coffee powder!

Final note: The nut for the cap over the burrs is reverse-threaded...use a wrench on top, and a flat-blade screw driver in the slot at the bottom of the motor shaft.