I had this happen to my Apple iPod mini. I didn't use it for a couple of months and the battery went dead. I tried charging for days and every available troubleshooting method and came to the conclusion that the battery just died bcause it worked fine before then. I made up my mind not to buy any other Apple product where the battery is not user replacebale. My iPod mini is just sitting here as a brick (with original protective film on top!). The screen wouldn't even turn on with charger plugged in. It is DEAD!!

At least on the N800, you could easily replace the battery. If you are able to see the charging sceen when plugged in, your device seems fine. Like someone else said, try a voltmeter to check the battery voltage.

Do you normally "turn-off" (shut down) your N800 when putting on the charger? I don't. I am assuming that you know to hold the power button for a while to restart it after shutting down.If the voltmeter shows bad battery, you might have to order a new one (made by Nokia will probably cost you $50-60 such as this http://www.nokiausa.com/link?cid=EDITORIAL_184164) or an OEM knockoff will be much cheaper but probably will be of very poor quality.

If the battery is good, take out the SD cards and try again. Also, did you install any software between the last two shutdowns?