How can I get ED-E back?

The first time I went to the fort in Fallout New Vegas, Boone and ED-E disappeared on me when I told them to wait at the 38 from the fort. After a while i returned to the fort, and all the sudden, ED-E floats up to me, but Boone is still gone. Later, i returned to the fort with ED-E and Veronica. As expected, they both fell unconscious and i finished off the legionaries. well, after the battle, veronica runs up to me, but ED-E is gone. I don't really care if he died, but he has some of my gear. I checked the 38 and took vault 22 to the fifth floor. nothing. if you have any ideas, let me know please. by the way, i found boone, dead, in front of caesar's tent. apparently companions can die on regular mode.