Use a nut splitter and buy a new nut
If the blade is a riht hans cut then the blade will have a right hans nut so it self tightens in use, rather than self loosens in use
Then thread below the nut gets rusty & caked in dirt so start with doing it back up tight and cleaning the thread
The spindle pulls out from the bottom so you can pull it out with the blade still on then hold the spindle in a vice to attack the blade nut .