I believe the Queen thinks that the death of his father caused Hamlet to go insane. This is what she says in Act II:
"I doubt it is no other but the main:His father’s death and our o'erhasty marriage"
This is what she says to Claudius, her ex husband's brother whom she married after king Hamlet's death. Polonius thinks Hamlet is mad because of Ophelia, whereas Gertrude thinks it is because of his father.