Your script almost certainly doesn't terminate (does it have an endless loop?), and since you've included it in your startup process, than never completes either. Since the desktop depends on startup (for e.g. graphical drivers), it will wait indefinitely for your script to finish.
You'll have to temporarily remove your script from boot sequence and modify it so that it forks a new process and terminates. In shell, that would be accomplished by writing
python stuff.py & instead of
python stuff.py. In python, you could use
os.fork() to achieve the same result. When you get your modified script running, include it back into your startup.