I was pretty sure that the companions could not die. I was doing the rockets red glare quest and got ambushed by a bunch of mobs. I cleared them out only to discover that Hancock that was my current companion had died. Not just hunched over in need of a stimpak but was dead dead. I tried reloading and the same thing happened. I decided to continue on in his absence and ended up finishing the main story line (railroad ending). Upon completion I noticed that hancocks corpse had followed me up to the top of the skyscraqer. Still dead. After those events I tried switching companions and none of them will allow me to select them. When I talk to them, they simply have their standard reply of wanting to go out adventuring again, but the dialog options never come up. I'm not sure how to fix this.