I made an extremely long animation, that took me hours!
Once I apparently "render" it, it doesn't want to load.
"This file isn't playable. That might be because the file type is unsupported, the file extension is incorrect, or the file is corrupt." Is what I'm told when I try to play the video that was "rendered".
I have FFMPEG downloaded onto my pc, can anyone help?

Try to play the file with vlc instead of the default media player on your os.