1995 Hyundai Accent won't start from time to time

My hyundai accent won't start from time to time, the lights go on and the radio works, there is no noise when I turn the key. I have taken it to a mechanic and he put it on the diagnostic and he could not find the problem. 2 weeks later it did it again and again he put it on the diagnostic and he kept the car for 4 days, still he said he didn't find anything. It starts when it wants to. I can't afford to keep taking it to the shop, my hours are being cut at work and $ is tight. I need a general idea of what it could be.