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?
More aboutbattery life completely
if the setting put monitor on sleep but leave hdd working that drain battery power,but to be shure that battery is still good charge it
If the battery is pretty old - more than a couple of years, that is not unusual behavior.
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.