Can't find my harddrive. My external worked fine for about a year, but this past week, my

Question

My external worked fine for about a year, but this past week, my powerbook no longer sees it. I've been using the firewire 800 connection and also tried the 400 connection, neither works. When I plug it in and turn it on, the lights still turn on. So it seems like the harddrive works and that it recognizes it is plugged in but my computer doesn't show it and I can't access files from it. Any advice would be greatly appreciated.