If the camera thinks another flash is attached it wont allow you to use the pop-up flash. This is actually a well-known issue and it can happen with lots of cameras.
The hot-shoe on the top of your camera has a couple of rails that hold an external flash. In these rails are a couple of leaf springs to provide just a moderate about of tension.
But hiding under one of these leaf-springs is a tiny little micro-switch. When you slide an external flash into the hot-shoe, it pushes down on that switch and that's how the camera knows you've attached an external flash. Over time, this switch can eventually get dirty and jam so that the camera thinks there's a flash attached... even when there isn't. When that happens, it'll refuse to pop-up the built-in flash because it thinks it would crash into the external flash (even though there is no external flash.)
Make sure the spring is able to spring up and you'll need to work the pin so it's able to spring up again. A tiny flat-blade jewelers screwdriver may do the trick. Also it may help to use a drop or two of denatured alcohol to dissolve any gunk that may be jamming it. Do NOT use any type of "oil" as that'll just be a dust magnet and the problem will return. Denatured alcohol, on the other hand, will evaporate so it'll be completely dry after you fix it.
Tim Campbell
5D III, 5D IV, 60Da
