Marlene Moseler , If still under warranty, return for repair/replacement/refund. Out of warranty contact the manufacturer(link below) for a replacement wheel with installation instruction, if no luck there slip into a local vacuum repair shop and have them take a look and possibly replace wheel with one off a parts machine. Good luck.
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http://www.sharkclean.com/products/0/all/19/shark-navigator-lift-away/