I used to have a 73 porsche 914 that I bought from a person who had left it sitting for the better part of a decade (it only had 43000 original miles :O!!!). It had a variety of strange things that kept it from wanting to start up. The battery almost always needed to be charged up because of a short somewhere. Give the battery a load test at your local parts store and rule that out. It had also been sitting for so long that enough water had accumlated in the gas tank and made it almost impossible to fire up. Try a can of HEET or something of the sort if there could be water in the tank. last and craziest, the engine wasnt getting air because a mouse had made a nest in the airbox and clogged it up lol.
Moral of the story, cars don't like to sit around and weird things can happen. Check stuff like spark plug boots, cylinder compression, intake pressure check, etc.
Lastly and possible most important. I'm not sure what the voltage for the fuel pump should be, but looking into its dying off. Maybe its not getting enough juice to get gas all the way to the engine.
2001 Audi A6 2.7t--------1982 Honda Accord LX 5 speed
Brilliant Black--------------Rust(ic) Beige