OK... One of my pet peeves is the words 'crank' & 'turn over' are useless any more because everyone uses them to mean something different.
[/rant-off] sorry, I just had to vent...

So, the first thing to do is decide whether it's lack of spark vs. lack of fuel.

Next time it happens, pull a plug wire & check for spark (d'ya know how?). Also, when you turn the key 'on' (not 'start') the fuel pump should run for about 2 seconds then quit. It's not very loud but you can hear it back by the gas cap.