There will probibly be a thin sheet of wood back which sits in a slot (All the way round) Get some wood glue and a plastic bottle with a spout. (The type you get in hair colouring kits is ideal).. Using this, squeeze the glue into the joints and corners of the wardrobe, wipe off the excess, leave to set, and Bob's your Uncle.....
That's the lazy way....
The real way would be to disassemble the wardrobe, re assemble using wood glue in all the joints.
CHUM
Chumley
October 2009