Moving this discussion to the InDesign forum.
Nille Glaesel which version of InDesign do you have installed? Also which operating system are you using? Are you able to open InDesign successfully?
What procedure are you using to open the file? Is the file supposed to be on a network share?
We've seen reports of files on networks being changed to aliases. I believe restoring the name to the .indd extension may allow the file to reopen.