When you say she looks dead is she still 'swimming/floating' and moving her fins? Or do you mean she is laying down on ground not moving!? If the latter it's likely she is ill.
yeah i agree with cichildlife i have been breeding guppies for ages now and i have never came across this problem but it sounds like the same thing i had with my balloon molly and it was cause she was in some sort of stress during the pregnancy and was releasing like little eggs on a white string as you can say but after i put her in a seperate tank with no lights she ended up given birth to a lot of little fry a few days later. if the black spot on the far end of the guppies stomach is more clearer and body more square then they usually are pregnant and they hang around the heater or filter hope this helps
the black spot by here rear is called the gravid spot, its the babies that are growing in her. If she is ready to drop she will have/show some of the symptoms listed above.... good luck, post pics if you can.