It sounds as though the file encoding was corrupted. You can see a similar long list of NULLs if you open an image or an mp3 file inside a text editor.
I don't know how Dreamweaver stores the files it is currently working on, whether they are cached in memory until written to the disk during saves, or whether they are saved in temp files on the disk (like MS Office).
Whatever the case, the file was open and in some sort of transitional state on your drive when your computer crashed. This transitional state is unrecognizable to your system.
This isn't necessarily a problem with Dreamweaver, though there are ways the program could operate that would make the files less prone to this sort of thing (Microsoft Office, for instance, writes just about everything to a temp file that is then dumped to the actual file you're working on, this is what allows it to display a list of "recovered documents" when you start it up after a crash).
Whatever the case, proper backups are always a good idea, and setting your application to auto-save at regular and frequent intervals is always a good idea.