If my memory serves me, that file is toast. I hope you have a backup.
Pagemaker can be really unstable, as Bob said multiple backups are the way to go. I save consecutive versions of my file as I work on it. 00pagemakerfile.p65, 01pagemakerfile.p65, 03pagemakerfile.p65 etc, so that if a file corrupts I can go back to a stable version.
Some things to try is to make a copy of the file in the Finder or File Explorer, and try to open the copy pf the file in PM by going to file/open rather than double clicking the file. Make another copy of the file and rename it one version down from your version of PM, so if you are using pm7 try to save it as filename.p65, if you are using pm7 try to save it as filename.pm6. Finally try to open it with a copy of Pagemaker on a different operating system, If you are on a mac try opening it on a pc, and if you are on a pc try opening it on a mac.
These options are a last ditch effort, and probably will not work.
It is not possible to install p65 in windows 8. Instead I have tried to open it with MS word with option to open any file. It opened. But the entire matter got dislocated. for example in page one some lines are from other remote page, some from other page and some from other page. My file is have 375 pages. Now to arrange it in order is a herculean task. Some body suggested to "place" the content from this dislocated word file into pagemaker. But the compu got hanged. My wonder is the techis discovered this much apps for anything and everything. But no app to rectify a corrupted file!
now i am trying to install p65 in a friends computer with WindowsXP and ther after I will try the suggestion given