My app keeps Crashing

I use a Mac and have recently put a mac build of my game on Steam. The Pc build however continually crashes on launch. Just after I select settings. I also can't seem to get any Crash report data to work out why. (if someone could detail how to do this that would be great)

Can anyone give me as many reasons why this may happen as possible and I will try to work through all of them over Christmas.

I understand this question is vague but thats kind of the point, I want as many 'what if's' as possible.