yenahs,
Generally it's recommended that you charge from a computers USB port as charging from an AC adapter doesn't have alot of control over how much 'current' is still flowing into the battery long after its charged. I understand it can be quite alot and could even destroy the battery or your mp4 player.
But with that said it's sometimes advisable to give an mp4 player a good charge of up to 12 hours before first use. Personally i'd say no more than 6 and from a computers USB port. If you do charge overnight make sure it's also on a USB.(so it's unlikely it will explode)
Just to check, does your mp4 players screen go black after you put it on to charge and do you see a "battery charging" animation when you do?
Have you tried charging the player on different USB ports? or on another computer completely?
When using your player it's wise to shorten the length of time the screens backlight stays on so it's not draining the battery prematurely. You'll find it in Sys Setup > LCD Set > Black >""