If it has dropped down on one corner then either the suspension rod has broken or just come away from the drum. Raise the front of the top deck by removing the 2 screws at the front. Have a look and see what has happened. If broken then you buy a new set of 4. They don't sell just one. Can't tell you the cost because I don't know which country you live in. Not that expensive to buy and easy to replace yourself.
Lincoln Appliance NZ, January 2012