I have a Nikon Speeedlight SB-800. I have not used it for a few months, and have always removed the batteries when not in use. On inspection one of the batteries was corroded and there was corrosion residie in the battery slot. I cleaned this up and installed fresh batteries. The screen came on as per normal, and if I put my ear against the unit, I could (just) hear it "singing", but there was no ready light and it would not flash. After a minute I took the batteries out and they were hot to hold.
Tags: Nikon, sb800, speedlight