Unable to connect to NXT using NeXTTool via USB cable
Joubarc's idea would probably be your best bet - try downloading the file to the NXT using the official LEGO software. If this fails, you know something is wrong with your brick. If it works, it's probably a problem with your program. Did anything major happen between the time it would work and the time it wouldn't? If so, this may be the culprit. Trying it on multiple bricks is also another avenue I would suggest. There is an official Mindstorms Help Section on the LEGO Message Boards located here. They have experts who will probably be able to correct your problem.