If you last charged it with the computer USB port, try charging with the supplied AC adapter instead. If you last charged with the computer USB port, try charging with the AC adapter this time. This will help isolate a bad AC adapter / power cord or USB cable / USB port.
It is important to note that if you are charging your Kindle via USB port on a computer, when many computer enters "sleep mode" the USB power is shut off at this time too. Charging is effectively halted - even though still connected. You should only expect to charge while the computer is active or when using the AC adapter.
You can also purchase an aftermarket DC charger. These are small plugs that are inserted into your auto's port or cigar lighter. They provide 1 or 2 USB connectors to plug in a USB cable. These devices are suitable to supply / charge just about any devices that uses a computer's USB port to power / charge a device. They cost anywhere from $5 and up and look something like this:
I hope this helps and good luck!