The reclining Buddha represents Shakyamuni Buddha (the historical Buddha) at his death. It is said that the Buddha knew death was approaching, and asked his disciples to prepare a couch for him in a grove. He lay there, reclining on his right side, facing west, with his head supported by his hand, as he passed into nirvana.
The image of the reclining Buddha is not supposed to evoke sadness, but rather encouragement that all beings have the potential to become enlightened and attain release from the sufferings which characterize the cycle of rebirth. The reclining Buddha's serene, composed, and restful demeanor (he is actually slightly smiling) a reminder of the serenity that comes with enlightenment.