Q:

iPhone app was working, suddenly starts crashing on startup, reinstall fixes, but why? 

Here's the situation:
-Had a development build of the app we just submitted on my phone, working fine for days (and previous builds worked fine for weeks). 
-Yesterday, I touched the app icon, the app launches then crashes out (I wasn't connected to my computer at the time, but if I had to guess, it was around or before when applicationDidFinishLaunching runs).  Basically, I briefly see my Default.png, then the app closes.  This happens every time I try to run it. 
-I restart my phone.  No difference, the app is still failing to launch.  All other iphone apps I have appear to be working fine. 
-This morning I reinstall the app from my development computer and all is well, it's working fine again.  No problems.  This is the SAME EXACT CODE with the same build parameters. 
This has me really worried, does anyone have any idea at all what might have happened?  It's like the app permanently broke itself, before it even hits any of my code no less :(.  I had a theory it might have had to do with an expired provisioning profile, but I didn't get any message then or when I reinstalled. 
I'd appreciate any advice/pointers/ideas people have! 

A:

Connect your iPhone to the computer, launch XCode, go to the Window menu, and select Organizer.  Click on your device in the left hand bar, and check your crash logs. 
That should tell you when it is crashing, and why. 
Actually, the crash logs should still be there from when your app WAS crashing, so even if you have reinstalled, you may be able to figure out what WAS going wrong.