If you have filemaker advanced, launch filemaker without attempting to open this file. Enable the script debugger and then open the file. The script that is closing your file will pop up in the debugger and you can then halt the script before it has a chance to close the file.
If "allow user abort [off] has not been used, you might also be able to use command period to halt the script if you can hit that button fast enough to halt the script before it closes the file.
I've heard of other ways to use an external reference to the file to get it to open without performing the "on open" script, but forget the exact details needed for that to work.
I don't have FM Advanced, so I tried command period. It took a few tries, but I finally got in. I'm still not sure why it wasn't defaulting to my main page, but I was able to disable the exit window script and save my database. Thank you!