Before I powered down, the battery was at 20%. When I came back 24 hours later, the battery light was flashing and it couldn't power up. So I would like to ask is it possible for the battery to deplete even when its complete turned off or did someone use it?
Self discharge is a known phenomenon for Li-on batteries.. however, 20% in one night seems a bit high. Are you sure your computer was completely shut off? If so, your battery is probably old and needs to be replaced.