Remove the shaft from the impellor to check for damage. If the ceramic shaft is broken or damaged in any way this will need to be replaced. The rubber bung keeps the shaft in place. Sometimes this remains in the base of the motor during removal. Reassemble
Guide to Water Gardening in New Zealand
from a Glenbogal Aquatic perspective
All you need to know about Waterlilies, Aquatic plants and Pond Care