The simple past tense of ‘loose’ is ‘lost.’
To explain, ‘loose’ is often confused with ‘lose,’ but they have different meanings. ‘Loose’ is an adjective that means not tight or free from restraint, while ‘lose’ is a verb that refers to the act of being unable to find something or not retaining possession of it. The simple past tense of ‘lose’ is formed by changing it to ‘lost,’ which refers to having failed to keep or to maintain something at a point in the past.