In Chapter Four, Crooks asks Lennie to think about what might happen if George does not come back from town. Since Lennie cannot answer the question (he does not "understand" its meaning), Crooks answers it for him:
They'll take ya to the booby hatch. They'll tie ya up with a collar, with a dog.
In other words, Crooks suggests that a life without George would be catastrophic for Lennie: he would find himself confined in a psychiatric hospital ("booby hatch") where he would endure all sorts of cruel and degrading treatments. This idea highlights Lennie's total dependence on George which is clear to Crooks and the other men on the ranch.
Crooks tells Lennie that he will go to the "booby hatch" because he wants him to understand how lonely life can be without a companion. This backfires, however, and Lennie becomes angry, demanding to know who has "hurt" George. This demonstrates Lennie's lack of understanding: in his mind, George would never leave him. Only some awful injury could prevent his return.
I think you mean "what Crooks says will happen to Lennie if George doesn't come back". In that case, Crooks surmises that Lenny would be locked up (take ya to the booby hatch) because he's "crazy as a wedge". Lennie takes more offense to Crooks saying that George isn't coming back than he doesn to the insinuation that Lenny can't lead a normal life, but Crooks makes his point either way.
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.