This is direct from the sansa FAQs at http://www.sansa.com/support/sansa_clip/faqs
02. Why won’t my Sansa Clip charge when I connect it to my computer?
Make sure that the USB cable is properly plugged in and use the original cable.
Computer in Hibernation mode
Make sure that the computer isn’t entering hibernate or sleep mode while the device is connected. The computer must be on in order to charge your player.
USB Port 2.0
Make sure your player is connected USB 2.0 port (1.1 occasionally causes charging problems).
USB Port location
Try plugging in different USB port (i.e. USB port in the back of the computer); sometimes the front USB ports have problem charging up the device.
Make sure the battery is fully charged. It takes approximately 3 hours to fully charge the battery.
Make sure to update firmware by downloading and installing the Sansa Firmware Updater
I hope this helps, and don't forget to rate the answer.