I think I read somewhere that if your computer doesn't recognize Kindle it might mean the battery is not charged - no matter how long you left it charging.
I think you might find even here on MR somewhere instructions how to charge it properly. I personally don't have wall charger *mine stopped working* but I had a problem of completely depleted battery and then there were some instructions how to charge it involving keeping the sliding on/off switch for longer time with some object (paperclip or whatever). I'll try to find the link when at home.
ETA: here is the link. Though it's for Kindle 2, this is what helped last year for my Kindle 3. Don't know if that would help in your case, but you can at least try.