The machine pictured here is a Nova, but the Mira is mechanically the same, so the instructions will work for those machines, too.
If your handwheel is stuck, and/or the needlebar doesn't disengage as it should when winding a bobbin, it's time to clean up in there.
First, unscrew the little screw in the clutch knob. You don't have to take it all the way out, just far enough that you can unscrew the clutch knob.
Just inside the clutch knob is the clutch washer, a ring with three tabs on the outside perimeter, and two on the inside. You may also discover gobs of thread wrapped around inside, as I did in this machine.
Once the clutch knob and washer are off, simply slide the handwheel off the shaft. If it doesn't slide, apply a loosener such as Kroil and tap gently on the wheel to work it in. Use a block of wood against the handwheel if necessary to gently hammer the handwheel off.
This machine has an additional piece to the assembly, a thin brass washer behind the handwheel. Slide it off for cleaning, too.
You'll notice on Necchi Nova models (and perhaps others?), the inside of the handwheel has a channel in it, and inside the channel is an oil wick. It may look like a filthy fuzzy dead snake, but it's supposed to be there, so don't pull it out thinking it's errant thread and lint. It's probably dry, so add some sewing machine oil to freshen it up.
Okay, now clean everything you've taken off the shaft. I generally use kerosene and a toothbrush to scrub off the greasy dirt. I use polishing paper (very fine sandpaper) to remove any residue from the shaft and inside the hole of the handwheel. Remember, to disengage the needlebar for bobbin winding, the handwheel has to freewheel on the shaft, so it needs to be clean and slick.
Once cleaned, put a drop of oil on the shaft, and another inside the hole in the handwheel. Smear it over both surfaces. TriFlo is a good product for this purpose, the little bit of Teflon helps keep things moving.
Finally, reassemble the pieces. On this machine, first the thin brass washer (cleaned, polished, and coated with a thin film of TriFlo)... then the handwheel... then the clutch washer. On this machine, the clutch washer is flat, but on many machines the inner two tabs are raised on one side. If so, remember the rule, "ears up," In other words, the side that those tabs protrude on is the side that is visible in the photo above. The easiest way to get that washer to stay put while you screw in the clutch knob is to tilt the machine and use gravity. If that's not possible, a little bit of oil on the inside of the washer will adhere it sufficiently.
When you've got the knob screwed back on, tighten the little screw and check whether the clutch is working properly. If it isn't (e.g., knob doesn't turn, or everything turns), take the knob off and rotate the washer 180 degrees, then try again. Theoretically the odds of getting this right the first time are 50/50, but in practice it's about 10/90.