Probably a low battery, or the terminals. Get yourself a volt meter, its easy to check that way.
http://www.sears.com/shc/s/p_10153_1...6&blockType=G6
A full battery will have 12.6 volts, 12.3 or less and its time to replace it. Of course that is without the surface charge. With the engine off, turn on the headlights for 30 seconds, then turn them off, then see what reading you get.
I had lots of trouble with the battery cables on mine. Part of the problem was the battery was not the right size. So it slide when I went around a corner and pulled the ground wire that goes to the fender off. Check the ground wire and look for the smaller wire that goes to the fender. If that one comes off it will also make it not start.