The battery on any SanDisk Sansa MP3 player will lose power every time you listen to music, cycle through files saved on the Sansa or otherwise use the player. Charging the player prevents the Sansa's battery from dying, and can also keep the MP3 player from freezing up as you use it. There are a variety of problems that could keep the Sansa from charging, and finding the exact reason is the key to being able to once again charge the player.
Check that the Sansa's USB cable is tightly connected to both the Sansa and the computer/MP3 player charger. Also, make sure the computer is turned on and not in sleep/hibernation mode, or that the MP3 player charger is plugged into a working electrical socket.
Use a different Sansa USB cable to charge the player. A faulty USB cable will likely prevent the Sansa from properly charging.
Connect the Sansa (via the player's USB cord) to different USB ports on the computer until the Sansa starts to charge. A weak or non-working USB port won't charge the Sansa.
Reset the Sansa MP3 player. Slide the player's "Power" switch all the way up to the "On" position. Hold the switch in that position for 20 seconds. The Sansa resets. Try to charge the player.
Install the latest firmware onto the Sansa MP3 player. Download the Sansa Firmware Updated (link in References), and then click the link in Resources for the specific model of Sansa you own to learn how to update the player's firmware.