Final Cut 10.6.5 has dropped clips and wont relink, including sound

I just finished editing a 6 minute video of many jpgs, pngs, .mp4 and a couple of audio .m4a tracks. 6 hours of work, and all went perfectly. Rendered all fine. When I went to share (ie export) 1080 x1920 file it dropped a number of the clips and jpegs, replacing them with a red "Extension not found". The sound track just went silent, nothing. It shows on the timeline as being there, can still be turned on and off with V key, but does nothing. When I tried to relink the lost clips it wouldn't even reload them, even after erasing the original in my media list. Its like it knows to not to touch them. And not just .png files, some jpgs, but not others. Trying to import it wont show half the files it had imported a few hours ago. Weird. And yet 15 minutes before exporting everything was working perfectly! Closed, emptied folders and trash for enough disc space, restarted everything, nothing helped. Running Big Sur 11.1.10 on older 4 GHz Quad core i7 with 32 GB 100 MHz DDR3. never an issue before and I've edited hundreds of videos on this setup. Emergency 911! I need FCP to work!