Specifically, in chapter four of Of Mice and Men, Curley's wife threatens Crook with her ability to tell white men that he, a black man, did something sexually to her. It isn't directly stated as such, but it is definitely implied.
Crooks stands up to her and is immediately put back in his place by her. She says:
She turned on him in scorn. "Listen, Nigger," she said. "You know what I can do to you if you open your trap?"
She closed on him. "You know what I could do?"
Crooks seemed to grow smaller, and he pressed himself against the wall. "Yes, ma'am."
And she finishes him off:
"Well, you keep your place then, Nigger. I could get you strung up on a tree so easy it ain't even funny."
Her use of the perjorative that ignorant whites use for blacks, and her reference to lynching, makes her meaning clear--all she has to do is say Crooks tried something sexual with her, and he would be hanged. He is, figuratively speaking, put back into the place society keeps him in.
At the end of Chapter 4, Crooks is a totally defeated man. He has had to give in to Curley's wife and he feels the fact that he is completely powerless. You can see this symbolized in the fact that he starts putting liniment on his back (his disability) after she leaves.
The reason his attitude changes is that Curley's wife has reminded him that he is a nothing. Up until that point, he had been sharing in Lennie's dream. But when Curley's wife treated him the way she did, he was reminded that he was only a black man and that any white person could pretty much do what they want to him.