Flash will not pop up
The flash no longer pops up, when it should.
The EOS 5D is a 12.8-megapixel digital single-lens reflex (DSLR) camera body produced by Canon. Released Q4 2005.
The Canon 5D is a full frame DSLR camera and has no built-in flash. So, I am assuming you are talking about an external flash (like a Canon Speedlite) which you attach to the hot shoe of the camera.
Tell us more about the flash (brand, type, etc.) and what settings you used when the issue occurred.