904 Trans suddenly stopped working

Well as the title says my 904 trans lost every gear. This is a project car, the car sat for years and years and years so I drained the old fluid and replaced the filter. 2500 stall was added and I filled up the converter before I installed it back into trans. Topped off trans and then started car. Continued to check and add fluid until it was finally completely full. Put it in reverse and reverse worked. Put it in 1st and 1st worked. Drove around the yard a little and everything worked. Backed car into the shop to adjust idle some more with car in gear and suddenly no gear worked. Not reverse, 1st, 2nd, or 3rd. Linkage is hooked up and working as it should. Finally ended up dropping trans pan and noticed a bunch and I mean a bunch of metal flakes. So I am completely confused how the trans can lose every single gear like that. The only thing that comes to mind is the first time I started the engine I heard a whining noise, but it quickly went away and never heard it again after that. Any ideas on why all of the gears are now not working? Picture of metal shavings attached. I was told that the converter should have spun freely before attaching flex plate to converter, but I had to use a screwdriver to turn it. Assuming that probably messed something up and cause the whine? Either way it's weird that it worked and then suddenly didn't.

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