Another theory is that the mayans went through deforestation and all of the crops died. The mayans went to another culture or died. Again this is just another theory.
Another theory is that the mayans were killed in a war against the spanish. The spanish had guns and the mayans were no match for them. But also this is just another theory.
But yet this reason of the disappearance is still meant to be a mystery. The mayan civilization was one of the greatest and yet to still be a mystery.