Grey screen Macbook air won't start Solved

I want to share my happiness and joy with you. My mac suddenly wouldn't start it showed a file with a question mark on a grey screen. I have tried everything possible an impossible (even created a bootable external hd) and was about to buy a very expensive replacement for my internal HD which my mac could not see and Diagnostic test showed the problem with it. I don't know why but I decided to vacuum it, literally used vacuum cleaner to get rid of dust on it and it worked. Now it works perfectly. Please, try it before dking anyting else.