Can't open .indd file suddenly!

Question

I have a large file that has been opening just fine for weeks. It's 300 pages and an .indd file. I have tried to open back-ups of this file and none with the same file name will open. I tried opening the file on another computer and it still would not open. 
I have opened other earlier versions of the file (different file  name) and other unrelated files with no problem. I opened larger files with no problem. I'm  thinking some how this file is corrupted.  i get a message when i try to open it that says: indesign can't open ***.indd." I just did some revisions that will take a huge amount of time to redo so I'd like to get this file opened somehow. I have a recent .pdf, possibly I could import that?  If not is there anyway to at least print a copy without actually opening it.
This all happened when i decided to create a Book file into which I would bring this file and then add a TOC, other front pages Glossary and index. I've been avoiding this step but thought it would facilitate bringing in everything together without creating one huge file.
Before taking the following step I  saved a copy of the original file on another hard drive.  Then I set up the Book and when I tried to bring my original file into the book it failed. After that I could not open any of my saved recent back-ups with the same name on other hard drives. I have three.
What if I try changing the name of the file before opening it, because I'm thinking it all has something to do with trying to bring it into the book file? Possibly try changing the file designation to .indd to .indb?
One odd thing, When I look at the file that won't open on the hard drive, it seems to have recorded the last time i tried to open the file as if I had actually opened it even though indesign gave me the "can't open" message.
Any  suggestions would be greatly appreciated.
Jim

Answer 1

This is a follow-up.  My question has been answered in that  did get to open the file as follows:
By trial and error (based on responses)  basically I was able to open the file as a copy from inside InDesign. There are three options in the lower left had corner of the window, I picked copy after trying the other two. Up until this time I could not find any idlk file.
Once I opened the file I backed it up and made another copy that I opened.  I went back to the original file and tried to open it from finder. It would not open. Still no sign of the idlk file.  But when I went back to open the work copy it opened but now the idlk file popped up in the finder  and it continues to do so every time I open any file associated.    This idlk file comes up with an icon that says "Locked"
My only question now: Is there a way to unlock this file from whatever it is linking to? Does it make any difference at this point?
I have learned this is a temp file of sorts that prevents changes to the original file. I would like to unlock the file so that it not connected to anything.
I have moved or deleted all files that it might be linking to.  I have shut down other computers and hard drives using the same network. The idlk file come up every time. It goes away when I close the file.  I have also deleted it while the file is open. I deleted all attempts at creating Book files. I'v been through preferences and checked Assignments. It seems like once you make a copy of a file, the original is always looking for the copy? Or the copy for the original?
So I'm working and making separate files for each chapter so that I can set up a book and have back-up and maybe create a whole new Book file that isn't linked to anything else but maybe that's not necessary.
I've been working with InDesign for a long time and never encountered this phenomenon before.
Thanks again for your responses.
Jim