Is the section line hidden?
Expand the tree and under section view, see if its section line is grayed (aka "hidden").
If so, right-click it and select show cutting line.
As to why...perhaps it was an inadvertent hide. Or, in doc options>View>section, are your label options set to none?
Thanks for both of your replies, seems that the layer visibility had been turned in a recent change to the template, hadn't thought of checking there.