sneubert described already the most likely result. However, the cause may be anything.

In a traditional console application you may use cout for tracking purposes. However, that will not work for an application with a GUI.
I prefer to use qDebug() you advantages when using together with Qt creator since the output can be found also in the application output window.

Best to start is having a qDebug output right at the beginning were your application starts. When you do not see even this output, it is possibly an issue with static objects and their initization sequence.

However, most of the issue you need to go forward step by step and you know teh details of your application.

Good luck.