Diagnosis
The unit has power, so the issue is either with the button itself or, more likely, the internal connection from the button to the motor has failed.
How to Fix It
1) Unplug the base. 2) Look closely at the plastic blend button. Is it cracked? Does it feel mushy or stuck down? Press it a few times with the unit unplugged to feel for a proper click. 3) Plug it back in. With the cup properly locked in place, try pressing the button firmly around its edges, not just the center. Sometimes the micro-switch underneath isn't being engaged evenly.
If That Didn't Work
This points to an internal electrical failure. The NutriBullet 1200 base isn't designed to be opened and repaired by users. If it's under warranty, contact customer service. If not, the base unit needs to be replaced. You can often keep using your existing cups and blades with a new base.
Heads Up
Before you decide it's dead, double-check the obvious: make sure the cup is clicked all the way onto the base (you should feel and hear it), and try a different electrical outlet. Sometimes a loose outlet or a tripped GFCI is the culprit.