D800 will not even boot
Okay, I had that problem with my HP less than a minute ago.
Maybe it is the same problem. See if the battery light comes on. Press the power button again and again if it doesn't start, for some time. It may show the boot screen and hang. Now, the battery must be charged. Shut it down directly and switch the AC off.
Now, switch it on and see if the battery light comes on. If it does, it is the same problem as mine. Now, turn the PC over and remove the battery carefully. There may be switches for that.
Now, plug in the AC and start it without the battery in. If it does, you're done. Now, plug in the battery again.