Typically you can probably lose only last changes for data and last changes for layout no saved.
When you exit record or layout data is saved.
Force quit cannot make a big trouble in this case.
For sure use Recover.
I would say that Force Quit can make trouble, if executed on the host copy of a FileMaker file (not the client one). The file may or may not be damaged by this kind of quit; and there is no sure way to tell, even if Recover does not find any damage. Best practices dictate that you should not use this file afterwards unless you really really have to. You should go back to an earlier file, Save as Clone, and Import all the data from the Recovered file. You would also need to reset any auto-enter serial id fields, to use the logical "next" number.
If you had been using FileMaker Server, then Force-Quiting a client would not cause this problem. At least not when the client is only running a script (as opposed to having Manage Database open, which is still a big problem). It is one of the big reasons to use FileMaker Server.
If the above sounds like a lot of trouble, then buy FileMaker Pro Advanced. It will let you turn on Script Debugger. You can then click the X to stop the script (hopefully), if you are caught in a run-away Loop, and have Allow User Abort ["Off"], which is the most common cause of this kind of thing. But if it is some other more serious error, I don't know; almost never happens (to me :-). Worth a try though, save damaging your file.