This is very strange. I was shooting macro with my 5ds with two speedlights. I had done several hundred photos at 1/200 f16 iso 100. Suddenly one image came out very overexposed and after that every image was black. I have to bring my speed waaay down and open up my aperature very wide to get any exposure at all. As far as I can tell, the camera is working fine without flash. What in the world is going on?
My first guess would be the flash is not firing properly or it is not syncing with the camera. Do you notice the camera does the same with the second speedlite only or that the flashes work fine with another camera body? Also, you may have checked this already but having experienced this myself, please make sure something like a sheet of paper or unintended light modifier did not fall in front of the flash head.