Two things come to mind:
1) The layer visibility for Hidden and Hidden Narrow has been turned off
Annotate tab - Format: Check the dropdown menu for Layers and make sure both Hidden layers are turned on.
2) Hidden Line style has been turned off on all your views. This seems unlikely, since it would need to have been turned off in every view (assuming there are multiple). Edit a view that should have hidden lines, check the Style settings in the lower right corner, make sure Hidden Line is on. If this is the problem, you will have to edit each view individually to fix it.