Without more information I would start with the basics and with the item unplugged:
Test the fuse
Check the spindle turns manually
Take the bushes out and check that they are seating properly and connected properly
Check that the bright shiny round bit the brushes run on is bright and shiny
Check the bearings are not worn
Test the cable is not damaged
If all the above are OK then the dangerous bit starts and you need to start trying to find where the fault is which may mean working with the supply connected which is where I would recommend you stop unless you really know what you are doing, have the appropriate tools and PPE.