Clean every component including the ball bearings, spindle, bottom bracket cup and inside the frame.

You can clean the parts by letting them soak in a solution of degreaser and water for a few hours, then giving them a thorough wipe down. Try to remove all of the old grease to avoid contaminating the new grease you're about to apply.

You may need to replace the ball bearings if they are broken.

Inspect the spindle. If you see significant pitting on the surface where the ball bearings would spin, you may want to replace it with a new spindle.