Not necessarily , you can try opening the back and exchanging the battery. It is pretty easy to do and the batteries only cost like 3 dollars.
If its not that then it will need to be serviced in order to work again , if you had bought it within the last year then you can get it done free from Nike .
http://www.nikewatchrepair.com/thats the link and instructions you do not need to fill out form if you do not have receipt just send a note with watch when you bought it and whats wrong with it along with your name email phone number and return address.
hope that helps