When I try to look at the vitals of my paternal grandparents, both of whom are deceased, everything that used to be there is now gone except for their one son, my dad. How do I get everything back?
Check the Change Log adjacent to your father's profile. Do you see details of changes, including (perhaps) "Relationship deleted" or "Merge"? Follow the thread of any changes and you may be able to reconnect.
If all the people in question are deceased, if you share the PIDs, someone else can take a look and perhaps help.
Here's the change log on my GGM where I did some clean-up work yesterday. There were duplicates of her and her parents.