Try opening the file while holding down 'Command' or 'Shift'.
If you have FM Advanced you could launch the application then start the script debugging tool, and edit it from there.
Sometimes, if you are really lucky, you can hit esc (windows) or command period (mac) to abort the script as the file opens if you don't have advanced.
Do you have any other scripts in this file?
You can create a can opener file that opens your file without triggering the OnOpenScript.
Create a new database file.
Give it a one line script:
and specify some other script in your original file that won't harm anything if you perform it like this.
Perform the script.
Go to your windows menu and you'll find your original file listed in the Show Window sub menu. Select it to bring it to the front.
You now have your file open and you can go to File Options and disable the script.
You might also want to consider this method so that you save more copies of your script while developing it: Saving Sequential Back Ups During Development