I am shooting tethered to a Windows 8 laptop which worked great until this morning when the tethered capture stopped working. The computer itself can view the D800 under "Portable Devices" in the Device Manager but Lightroom 5 cannot.
My guess is that Windows is somehow taking control of the camera and not allowing Lightroom 5 to access it.
We tried to tether using different USB ports, different USB cables, and a separate D800 camera all with no luck. Also, the tethers work fine on another Mac. We also tried to restart the Windows laptop with no luck.
My first guess is that a Windows update installed between the last time the system worked as expected and the first time things didn't work is the culprit. Your comment above confirms this.
If you can't keep Windows rolled back then you may want to contact Adobe and let them know of the incompatibility. The more complaints/reports they get regarding an issue the more likely they are to develop an update for LR5 that fixes the problem from the application side.
So, it seems (so far) that the issue has been solved by changing my Toshiba Laptop's USB port settings from 3.0 to 2.0. The USB Cable itself is a 3.0 so I don't know why this works but, I'll take what I can get at this point.
Is the camera in a different mode from normal? The most likely cause would be that the camera is connecting as a storage device rather than as a camera. I don't know the D800 specifically, but this is typically linked to either the camera's operating mode or based on if the LiveView shooting mode is enabled or not. (Not sure if Nikon calls it LiveView, but the mode where you see the shot on the LCD.)
I'd try toggling these settings and see if that fixes it. If not, perhaps try the standard troubleshooting steps of rebooting the computer and turning off the camera, pulling the batteries for 30 seconds or so and then putting them back in and reconnecting everything.