The lifecycle of activities is explained in extensive detail in the javadocs of the Activity class. I seem to recall pointing you to it more than once. If, after all these months of programming for Android, you still haven't internalized it, then, frankly, you're doing something wrong.
But i have installed the all again.even then oncreate doesn't run.
That sounds very odd, not to say highly unlikely. Stopping an app (via the Settings) or uninstalling/reinstalling it will definitely cause an app to be terminated (and thus cause onCreate to be run the next time it is started). If you can reproduce that that is not happening for your app, then that would be a grave bug in Android.