copy the frames/layers from your 3 hour work and paste that into a new fla and continue working.
then learn from your experience. periodically save your work with a new name.
i recommend appending a version number so if you're working on a project name xyz, you save xyz_01.fla, xyz_02.fla etc.
in the publish setting i like to assign the published files to all have the same name, xyz.html,xyz.swf,xyz.exe etc.
I have multiple versions saved. And each different file still acts the same way. I tried to copy it as you suggested and I'm still having the same issue.
that doesn't make sense.
if flash crashes while you're working on version _x it makes sense that version _x is corrupt. but it makes no sense that another version would be corrupted.
I know, I have no clue what the problem is. Possibly something wrong with my tablet or program connection? All I know is it's lagging everytime I'm trying to draw.
maybe your tablet drivers need to be replaced. update them.