All of my layers including layer 0 are missing from my Layer drop-down list. Only my current layer is listed but the Layer Properties Manager shows that all my layers still exist.
At the bottom left corner of the Layer Properties Manager is an option named “Invert Filter”. Should you have accidentally ticked this box and been using the default “All” filter option, in the Layer Filter tree, it will invoke the opposite of all which is none!
This wouldn’t normally cause an issue with the layer drop-down list unless you had also selected the Layer Settings icon at the upper right corner of the Layer Properties Manager and then ticked the option to “Apply layer filter to layer tool bar”. All of these settings combined mean that you end up with just your current layer displaying in the layer toolbar list and it does occur as a support query from time to time.