The woodruff key on the shaft has sheared and the hub is no longer connected to it. To check, try locking the disc with the button used to stop it rotating- if you cannot, the key is broken
depending what type of disc you're using depends which way round the locknut goes onto the spindle, one side the nut has a raised part the same diameter as the hole in the centre of the grinding disc and is flat on the other side, so, flat side for thin cutting discs, raised side for grinding discs. if the raised part is used on a 1mm thick cutting disc it won't lock tight onto the disc.
Have you considered that the securing nut in the centre of the cutting disk might have become loose?
A Nut issue...
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