When you trying to open your flash builder ,sometime it shows that window appears and then disappear, and your flash builder doesn't start. So, here is the solution for this.
You just have to delete the .lock file in worksapce.
Path to get this .lock file is " \projectName\.metadata ".
Just go to the workspace where you kept your project and inside your project their will be .metadata folder. Inside this folder .lock file will be their just delete it and try to run your Flash builder again.