I have been using blender 2.79 and recently downloaded 2.8.
I was modelling a scene in blender 2.79 and when opening it up in 2.8 most of my objects are greyed out.
Everything is enabled in the viewport, nothing is hidden.
One object is still visible but the rest are not.
I have tried using the filters, changing the visibility in the collections to restore visibility.
The weird thing is, blender is saying that they aren't hidden but can't be seen in the renderer or in the viewport.
Objects will be grayed out in the outliner also when they are linked to an object from a different collection.
Let me show you:
The object Foil door.001 is gray because it is in the main scene collection and at the same time it is linked to the empty DoorHinge.000, which is in the collection Doors.
Therefore Foil door.001 is actually visible in the outliner twice (see image).
Both entries refer to the same object. The gray entry only indicates the relation to the empty DoorHinge.000.
I hope this also helps.