Replaced Crankshaft sensor but car won't start

I parked my Jeep then got up the next morning and it wouldn't start. I had it towed to a shop and they replaced the the crankshaft sensor. It ran beautifully for about three days. I went into a store and came out to find that once again it wouldn't start. It has no signs of anything wrong with it. It just starts once then won't start the next time. It will crank over just fine but no spark going to plugs. Crankshaft sensor was replaced and it ran for a few days but then it idi it again, will crankover but will not fire. I beleive the crankshaft sensor is part of the problem but not the root of it. Any suggestions?