Once low, OUT stays low until 1) IN is above the threshold voltage AND 2) the latched output is reset. When IN is first applied (say, at powerup), the latch will start with the output low and will need to be cleared.
To clear the latch the CLEAR pin needs to see a low-high-low transition with at least 1µs high duration.
Please note that this part does not have rail-to-rail inputs or outputs, due to the internal structure the comparator could saturate and be unable to switch if IN = Vcc. Please follow the Typical Operating Circuit on page one of the datasheet, which includes a resistor-divider. Aim to keep Vcc greater than IN by at least 0.6V.