Also, if its stoney flat, try plugging it directly into a wall charger rather than trying to do it through a PC. I can't speak for sure on modern gen iPhones as I've not had that problem, but I know for definate most older gen iPods, when completely out of juice, wouldnt actually initiate a charge off of a PC USB.
And, like Red Light says, leave it for a good while. When I got my 32GB 4 new, Carphone Warehouse had to get it from a warehouse somewhere with a 14 day lead time, so I guess it had been sat around for a bit which is never great for a battery. It took the best part of two hours before I had any sign of life out of mine.