I fear you learned a lesson the hard way. Wise to press ctrl-s / cmd-S reasonably often to make sure your BPN file is not as far out of date was yours apparently was.
This article:
http://www.archicadwiki.com/AutoSave
gives the default autosave location for Mac... but I just checked there, and I have no autosave files, so apparently the files are erased after a normal exit? Maybe it is different on Windows.
This thread may be helpful, but equally unsatisfying:
http://archicad-talk.graphisoft.com/viewtopic.php?t=7277&start=0&postdays=0&postorder=desc
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB