Diagnosis
That's the machine's float sensor not recognizing the water level. It's almost always a simple air bubble trapped in the water line from the initial setup.
How to Fix It
This is a quick one. First, unplug the machine for 30 seconds. Plug it back in. Now, lift the water reservoir completely off the base and set it down again firmly—you should hear a click. Finally, press and hold the 8oz brew button for 3 seconds. That usually purges the air lock and resets the sensor.
If That Didn't Work
Check that the reservoir is seated perfectly square on the base. Sometimes if it's even slightly crooked, the magnet on the bottom of the reservoir won't align with the sensor in the base. Also, make sure you're using the official Keurig reservoir; some universal ones don't have the right magnet.
Heads Up
Don't just keep trying to brew with the light flashing. The machine won't heat without recognizing water, so you're just running the pump dry, which can damage it.