When you say the battery won't charge, what exactly is happening? Are any of the lights coming on? Is it flashing red? Is it turning green immediately?
If there's no lights: it's likely the adapter is fried (the black box), you can buy another from any good computer/electrical store, but ask if you can test it in store before you buy in case it's the green cradle that's gone (see below).
If the light is flashing red: grab some rubbing alcohol or nail polish remover and clean the silver terminals on the charger, battery and inside the Pleo (little springs).
If the light is going green immediately: take it off charge, and put it back on, keep repeating as many times as you can until you are bored out of your brain or at least an hour has gone past

Then try it in Pleo.
If none of the above helps but you are getting lights on the charger: the battery is absolutely dead beyond repair

And you would have to risk buying a new one.
If the charger is absolutely, completely, and utterly dead even with a new adapter: Again you'll have to decide if it's worth buying a new one.
It is more likely to be a battery problem than a Pleo one, but we certainly can't rule that out.