There is two sides to every equation, and you need to figure out what side your problem lies upon. Either it's the positive, or the negative. That is where you start with this. Get out your jumper cables to the battery and connect up the negative first to the negative on your motors. Does it run? If so, go back on the negative towards the battery and find out where it fails.
If not, connect up a jumper cable to the positive side of your battery, and to the motors. Does it run? That proves the negative side is good, and you should look at the positive feed to find your problem. 50% of your problem is now solved, it's one side or the other that is not working.