I'll start with the obvious:
1. Have you tried restarting C4D?
2. Check that viewport solo mode is not turned on:
- under the menu: Tools > Modes > Viewport Solo Off
3. Check the stop lights next to each object and make sure you didn't accidentally swipe them all off - this is easy to do by holding Alt+Shift and dragging over the stoplights next to any of the objects.
4. Check your layers and make sure they aren't included in a layer that was set to hidden.
If all else fails then try selecting all the objects in the object manager, copy, create a new scene, save that scene in the same directory as the original and then paste. All materials that are applied to objects will carry over but you will have to do the same process for other materials and render settings.