Why can’t my MacBook find the hard disk?

My macbook has been acting up and when I start it up, the "install Mac OS X software" window comes up. So when the select a destination part comes up, nothing is there. (When usually the Macintosh HD is there). Can you please tell me what this means and how I can fix it? thank you
P.S.- My warranty expired already so I want to try and fix it myself first.

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