Try turning off your d800, then remove the batt, then turn on the cam without a batt installed for a few seconds, then turn off the cam, then replace the batt, and turn on the cam again. This might help clear a confused memory chip.
I would then try a two-button reset to restore the d800 default settings. Tamron lenses can mess things up. The camera settings can be restored to default values by holding the qual and +/- buttons down together for more than two seconds (these buttons are marked by a green dot). The control panel turns off briefly while settings are being reset.
And since you are in the heart of the civilized world, if nothing else works, remove your third-party Tamron zoom lens and go fetch another used and inexpensive compatible Nikkor prime lens and try to get your d800 to function properly after repeating the above steps. good luck.