I'm playing on non-hardcoe mode. After finishing Boone's quest by making him remorseful over bitter springs, I left him in the lucky 38 with a LMG, Combat Armour Reinforced and Combat Helmet Reinforced and some sunset sarsaparilla. After a long time, I decided to have him join me to get cazador eggs for red lucy and I found him dead at the dining table of the lucky 38 presidential suite. All companions except Rex and Tejada were in the Lucky 38 at the time. How do I resurrect him back to how he was?

Load an earlier save and see what he is doing.

If your on the PC, then you can use the ressurect command in the console.