Sounds like a script set to run "onFirstWindowOpen" is trapped in an infinite loop.
Press the escape key (Windows) or command-Period (Macs)
to abort the script that is running.
This can fail if the script uses Allow User Abort to keep this key from aborting scripts. If that's the case, you can either launch FileMaker Advanced without opening the file, enable the script debugger and then open the file in order to bring up the script in the debugger so that you can abort it from the debugger, or you may be able to create a new database file and run a perform script step from this new file that performs some "safe" script in the original file as a way to force the file open without tripping the OnFirstWindowOpen trigger.