The batteries are good for a few hundred cycles.. More if you deep cycle them, less if you don't. ['swhy rockbox has the deepcycle option] - So.. it's possible you're batteries are dead. No biggie,
just pick up a set of NiMH battries at Cosco or Radio Shack or
similar - get the biggest capacity AA you can find (Cosco has
some 2200mAh ones now) and replace them. You just pop the
side covers off, one on each side. You'll see the batteries underneath.