If you have a Dell laptop, and out of the blue you get the error NTLDR is Missing when you turn the machine is turned on, followed by an Invalid MBR error on subsequent reboots - DO NOT PANIC
Chances are, you accidentally pressed the Media Direct Button (looks like a house on my laptop) instead of the Power Button. Now if you have a Dell standard install, this will probably load the Media Direct application - but if you have done a wipe / rebuild, and not set up the Media Direct partion as expected, then you will likely get NTLDR / MBR errors, and an unbootable machine.
When you press this little button, the system rewrites the MBR on the primary disk, and causes the system to boot to a different partition (one which theoretically should contain the Media Direct application). If that partition does not exist, you get an apparently bricked laptop.
The good news is, that if you press the button again, then the changes to the MBR get reverted - may need a subsequent reboot - and you should be able to get back into windows :)
Dangerous button :-/