launcher says its loading, finalises, closes and then nothing happens, minecraft doesnt open

Put the summary of the bug you're having here

What I expected to happen was...:
miecraft 1.6.4 modded to open

What actually happened was...:
the launcher closed as if was about to open the game then nothing happens

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...