macbook pro issue pls help

my macbook pro woks just fine until when it runs out of battery it goes into sleep mode but when i plug the power cord it does not do the restoring that it usually do, nothing happens so i press the power button but it reboots and my work is gone it suppose to restore from its state before it sleeps thats the normal thing right, it happens a few times already , after the booting the time and date is reset. what would be the reason for this issue, is there any fix for this problem and whats the reasons why this things happen?

Mac os X 10.5.8
processor 2.26 GHz Intel core 2 Duo
memory 4GB 1067 MHz DDR3
startup disk leopard 10.5

If it runs out of battery, then it's not technically sleeping, is it? It's dead.

A simple fix would be to not let the battery run out of juice.

but sir i just wonder why it does not restore after it runs out of battery before my mac restore from previous state after i plug the chord. is this a battery issue and should i be worried with this problem?