My sourdough starter straight up died on me Tuesday morning

I was getting ready for my weekly bake and went to feed my starter, Frank. He was completely flat, smelled like old gym socks, and had a weird gray liquid on top. I think my kitchen got too cold overnight because we left a window open. I tried to revive him with a bit of warm water and fresh whole wheat flour, but after 12 hours, nothing. Has anyone brought a starter back from the brink like this, or is it time to start over from scratch?