If your using a Laptop, it may be your ports! The same exact thing happened to me, the whole discussion is here:
Port Problems!
Basically your laptop port dont have enough power to power the Arduino, it will work for a while then one day nothing! My fix was a desktop, but you can buy a port hub with an external power source! I spent about 2 1/2 weeks troubleshooting this problem! Here is a link to the type of Port Hub I am talking about,