Turning the crank in the rewind direction should tighten it, not loosen it, unless it was on the verge of coming apart already. If nothing else is wrong, you may be able to just screw the knob back in, tighten it by turning clockwise, and rewind normally. You must of course push the rewind button on the bottom of the camera before rewinding.
If for some reason that doesn't work, you will need to open and unload the camera inside a completely dark room or changing bag. You would need to open the back by pulling the rewind shaft up by inserting a screwdriver or pick into the shaft. Then you can rewind the film into the cassette by hand (again, push the rewind button first). With the film back in the cassette, you can turn on the lights.
To screw the rewind knob into the shaft, use a screwdriver in the slot of the shaft to keep it from turning, and screw the knob in by hand. Again, this should be easy since you turn the knob in the rewind direction to tighten it.