Yep, happened to me too once. Lost 300 hours of progress.
That taught me to check where my games save their files before wiping hard drives.
The silver lining is that even though your save files are lost, the knowledge you’ve gained is not. you may have to start from scratch, but you’ve learned how the game works, what to do, where to find resources and NPCs, etc., so getting back to where you were is a lot faster than it was on your first time.
I suggest doing what I do now and backup your save files regularly on a different hard drive; CodeMage provided you the info on where to find the save files. (The actual location of your Steam files may be different, depending on where you’ve told it to install your games and stuff, but you’ll find it.)