Why does my Mac sometimes turn on and sometimes not?

Since 2 weeks my Macbook Pro is sometimes turning on and sometimes not. When it does not turn on, it has a flashing file with a question mark on the screen (when I go to disk utility it does not recognize my SSD hard disk). I went to the genius bar, but unfortunately in that moment it did turn on, so he couldn’t really do his magic, but I was told it may either be the operating system or the hard disk. At home I reinstalled the operating system, but the problems continues. Does this mean, that it is definitely the hard disk? Then I could try to fix it myself. Or could it be anything else and I better bring it for professional screening and repair? Is that even possible that the hard disk sometimes works and sometimes not?