We just bought a D810 and haven't been able to get the built-in pop up flash to work.
When we open up the flash, it fails to charge. We also can't assess the flash mode with the flash button and the main command dial on the camera - nothing happens.
Any suggestions?
Fails to charge means what? Is the flash Ready indicator on? That's the Lightning bolt symbol at lower right end of the Viewfinder, #17 on page 6 of the D810 manual. It should be On if the flash door is open.
The flash mode is menu E3, page 331. TTL, Manual, or Commander.
The flash sync modes are page 191. You say "nothing happens", but that seems impossible. If you hold the flash button (#2 on page 2), you should see the sync menu in the top LCD (page 190 & 191).
If you open the flash door, it should flash with the shutter button in any camera mode A, S, P, or M. Only two reasons I can think of that it will not flash.
1. the shutter commonly wont trigger if it cannot find focus. If the shutter does not trigger, then flash does not trigger either.
2. If you have Auto FP on (menu E1, page 321), and the shutter speed is faster than sync speed (1/320 second). The internal flash cannot do Auto FP mode. Try a shutter speed slower than 1/250 second.