It sounds as though there was a crash of some sort while the file was open, and there is a "lock file" present. (It would normally be deleted automatically when the InDesign file is properly saved and closed.)
Navigate to the folder where the InDesign file is stored and look for a file with the extension ".idlk" Delete that file and try again.
I cannot seem to locate the indesign file at all. I can only find the PDF I saved it as on my computer to share with others.
Again, maybe my inexperience with computers is really hurting me here.
InDesign can only "Place" a PDF as an image, not open it for editing, unless you have a third-party plugin.
You could export the text and images from the PDF using Acrobat pro (you might need to save as HTML, then open that in Word to get the text to look good) and then recreate the file from the ground up, using the PDF as a guide.