It is hard to get N900 to charge totally dead battery. You might do some searches on the forum and wiki about flat battery recovery. One of the options is also use external charger or some Nokia Dumbphone to charge the battery. Some people have managed to revive flat batteries by leaving them in phone connected to charger for full 24h.

Edit: adventurous person could charge the new drained battery with the old worn battery by connecting the + to + and - to -, but this ofcourse runs the risk of blowing up both of the batteries on your face if you screw it up. Basically you just need to do it for short period of time to get the new battery just out of flat level so you can do rest of the charging on the phone.