After charging my MP3 player it appears fully charged but it will only play for about an hour and the battery runs down. i have recharged it again, same thing. now what?

Taken from the Quickstart User Manual

LED Light 1 (L1) - is by the earphone socket
LED Light 2 (L2) - is by the USB connector

Charging: Please switch on the player before charging. (Push the on/off switch to 'on' )

When charging the battery for the first time it is recommended that the player is charged about 8 hours. Connect the player to an active USB port using the supplied cable. The computer must remain on during the charging process.
Recharging: Recharge the player will take about 3 to 4 hours. When it's charging, L2 will turn on in red, when it's fully charged, L2 will turn to green light.
When playing music, L1 will shine.

Hi
Check the Manual
User’s Guide


Hay Don't worry it will work
Note: Some low-power USB ports may not be able to provide enough
charge power. Also note that rechargeable batteries may
eventually need to be replaced as they have limited charge
cycles (which vary with use and settings).

Li-ion Battery
The Sansa e200 MP3 player comes with a replaceable and rechargeable
lithium ion battery. To use the player for the very first time, remember to
fully charge the Li-ion battery. You can charge your Sansa through the USB
port of your computer, a car adapter (not included) USB port, or AC adapter
(not included) USB port.


Please execute the following instructions for charging the battery using
an USB port.
1) Connect the smaller end of the USB cable into an available USB
port on the computer
2) Connect the larger end of the USB cable into the dock connector of
the player
3) The computer must be on for most USB ports to charge your
battery. Your Sansa will show connected and the yellow charging
indicator will appear. The battery level indicator will blink while
charging. You will not be able to play while connected to the
computer USB.
4) To stop recharging, disconnect the cable.
The battery can also be charged using an AC adapter (not included) or
a car adapter (not included) and you will be able to play your device
while charging with one of the adapters. The battery will reach
approximately 70% charge in 2.5 hours and full charge in about 3.5
hours. It is recommended that you fully charge your Sansa e200
before the first use.

charge your player and reset and try again if not works
then replace battery of your device that is damaged

most of the time when your plugged into the pc it will charge see if this is true for your player if so leave it on the the pc for a few hours also drain the battery fully then charge for a few hours

Try resetting the device and charging it again. You may have to charge it for a few hours though.

The player's battery is recharged while the player is connected to a running computer. It takes about 4 hours to fully recharge.
While the player is connected to the computer, do not turn on, restart, wake up from sleep mode or shut down the computer. Doing so may cause the player to malfunction. Disconnect the player from the computer before performing any of the above functions to the computer.

Your battery is recharged when connected to your computer. Turn the player off and connect it to your computer via the USB cord. The computer must be turned on. Initial charge should be for 10 hours with subsequent charges 3 to 4 hours for a full charge. A quick charge of 1-1/2 hours will give you a 70 percent charge. A fully charged battery will last up to 15 hours under normal use.

it doesnt need to be turn on while charging.. you can see an indicator stating the unit is charging when this indicator stops it means the mp3 is now fully charge..

Connect the supplied AC/DC adapter/charger to the 5V DC jack on your hdd and the power supply. The AC power icon will indicate on display showing that the Jukebox is now adapter operated. It takes 4 hours to fully recharge the hdd and once the battery is recharged fully, the jukebox can play up to 10 hours of music on the move

It is just like that because it isn't done recharging. After a while it will turn back to what it's real power level is. You should let your MP3 player recharge for over an hour at least to have it fully recharged.