When I open an app it immediately closes back to the homescreen. How do I fix this?

There is no "quick fix" for all. It depends on what's causing the crash.

Delete and install the app again

Reboot iPhone

If there is a conflict with a running service or app, then that could be causing it, as well as memory usage.

It could also just be crap coding by the developer.

Is this the same app all the time or every app? If it is the same app then it might just need to be updated by the developer. Try resetting the iPhone and see if that works, or even delete and then redownload the app.

Sometimes simply closing the app and reopening will solve the problem. Double tap the home button then tap and hold on the app until the red "-" appears and close it then try relaunching. Also try restarting the phone to see if that helps anything. If not, then this app probably has incompatible code with the version you just updated to in which case the developer must fix it.

Ken

What does 'privately developed" mean? If you got it from the app store using the correct iTunes account, it should work. Try logging out and back in to your iTunes account on your iPhone. Also, try downloading/installing any free app.