Not knowing the recipe nor brand of ingredients I will hazard a guess.
You left the crock pot on warm.
The macaroni continued to absorb the sauce which broke the cheddar? cheese.
You were left with mushy pasta and tiny curds of cheese.
Like I said just a guess although an educated one born of experience lol.
Agree with @MillionsKnives
cannot be saved.
Sounds like a PB&J kinda nite ;-)