Double check to make sure that your machine isn't sleeping after a certain amount of time, say an hour. It won't render when the machine is asleep. Just turn off the monitor at the front and let the thing render away. Another possibility is that the scene is so complex that the machine is still thinking about how to render that next 1%. I had one project that took about 2 days to render. There were many times that I thought Bryce had crashed but it hadn't. Try taking out non-essential things or try turning down the transparancy and reflection on non essential things. This will help speed things up.