The adverse effects of running without your radiator cap, you've already experienced.

The cooling system is designed to run under pressure.

This is the reason why you should not fill the coolant reservoir completely since once the mixture heats and expands in your system, the excess will go to the reservoir and once things cool a bit, it sucks the excess back into the system.

This will not work effectively if there is no pressure.

Replace your missing cap as soon as possible and be careful to avoid the same situation in the future.

Hope this helps.