InDesign won't open a file which I last used 20hrs ago.

What you can always do is try to open the file in a different InDesign version and let it convert online. Sometimes this helps. Then again, there is a whole page vom Adobe dedicated to this problem, maybe you will find help there: 
https://helpx.adobe.com/indesign/kb/troubleshoot-file-issues.html

Tell us more about your system: Please tell us what operating system (exactly) you're running. Please tell us (exactly) what InDesign version you're using. Is this this the only file which has a problem: Do others files work fine?

What error do you get?