One of my shared layers is now uneditable. I cannot select any of the elements in the layer, it is locked and I cannot unlock it, and when I click to edit it, the pencil icon has a line through it.
Perhaps you were using a Pro trial that expired? Editing shared layers is a Pro-only feature.
If you purchased OmniGraffle 6 from the Mac App Store, intending to upgrade from OmniGraffle 5 Professional, you need to “purchase” the free upgrade as an in-app purchase. You can do that from the OmniGraffle menu.
If you’ve already done that, you can try restoring your purchases.
If you bought from the Omni Store, make sure you have a license of the appropriate type installed.