Does anyone know a way to recover a FrameMaker file after FrameMaker crashes?

Step 0 is to not try without a plan.
And based on your remark about the backup file, we may be too late for that.

Preserve all the files as-is, including any temporary files that FM had created (which may not be in the document directory - I haven't needed to do this since FM7/Unix, so I'm not completely sure where to look).

What had you elected, if anything, for auto-save in FM?