I believe you meant what Crooks says to Lennie if George does not come back. In Chapter 4, George has already gone with some of the ranch hands to the city and his only instructions to Lennie were to "not get in trouble."
Crooks, however, has *much* to say to Lennie about what would happen if George should fail to return. First, he lashes out, cruelly telling Lennie what is probably true: "Want me to tell ya what'll happen? They'll take ya to the booby hatch. They'll tie ya up with a collar, like a dog."
Seeing the distress in Lennie's face, Crooks eases up. He projects his own loneliness on to Lennie: "S'pose you didn't have nobody. S'pose you couldn't go into the bunk house and play rummy...A guy needs somebody -- to be near him.
Crooks is right on both counts. Lennie will suffer, in fact die, without George's protection. And George, having lost his bosom friend, will learn Crooks' loneliness.