Nodes are deactivated/inactive/greyed-out and cannot alter value I have nodes but somehow they are greyed out and I am unable to change values. Did I press a hotkey accidentally? If so what was it?

An example is attached below;
This issue could have been caused by linking in an object from another scene. When you link in an object, "ghost" materials are created - completely identical to the materials from the original file, but uneditable.
If this is the case, then simply append objects or materials themselves and you should be able to edit the nodes again.