For reinstalling ffmpeg, try to locate a file ffmpeg.exe in
either your Sketchup Plugin directory
or a folder C:\FFmpeg
In principle, you should know or remember where you did put the file. Otherwise do a Search file in Windows.
When it is located,
delete it
get a fresh copy of FFmpeg.exe
put it somewhere on your PC
Animator will find it if you put it in C:\FFmpeg or in the Sketchup plugin directory, or will ask you to locate it again
Now, if you still get an empty mp4,
Do you see the MP4 button in Red in the result dialog of the video generation?
Are you rendering, or jsut generating the 'Natural' mode (no rendering)?
Can you open the Ruby console before generating the video and check if there are erros (there should be traces)