This just happened to me and i have had no luck getting it back. Sort of alarming to have your entire CSS disappear. I'm using Chrome, but I've cleared my cache, logged out and back in, tried a completely different browser and it's still gone.
Site URL: https://www.windandfoglandscapes.com/
I have quite a bit of CSS code that I entered and it suddenly disappeared. The code is still working however. I've logged out, cleared my cache and it's still missing. Has this happened to anyone else?