To become insensible; to lose the ability to think rationally or logically; to be unable to reason. A possessive pronoun can be used between "lose" and "reason." My brain was so addled with drugs that I completely lost reason. What is going on with the boss's decisions lately? It's like she's lost her reason.
lose one's reason
Fig. to lose one's power of reasoning, possibly in anger. I was so confused that I almost lost my reason. Bob seems to have lost his reason when he struck John.