Drawer bottoms are very weak as they are usually made from hardboard, and the slightest bit of overloading causes them to bow downwards. If the drawer is of a self assembly type it should be possible to dismantle it using a screwdriver to take the hardboard out. Take the piece of hardboard to a friendly hardware shop that supplies timber and ask them to give you some plywood that is exactly the same size and thickness - its much tougher stuff and should sort this out.
John
January 2008