This morning I have a display problem when I load my assembly (see attached screenshot). I don't think I've changed anything in the options or in the tree. I'm SolidWorks 2017 SP5.
A colleague has loaded my assembly and it has the same display problem.
It happens to me from time to time too. When you right-click on a part in the assembly, you can click on modify the transparency or something like that.
Like ACCobra, I lean towards a display state set inadvertently (see the gray and white triangle icon). All you have to do is open the different levels of subsets to find where it's happening.