SFK files only hold the visual representation of the audio waveform and are essentially useless from a user perspective.
It sounds like somehow the source clips got deleted. Perhaps you inadvertently deleted them (when I first started editing in Vegas I accidentally deleted a few clips out of the Explorer view thinking I was only removing them from my project, but nope, that deletes them from the disk).
You’ll need to copy your source clips back to the folder from a backup (or maybe check the recycle bin?) and then your project should open normally.