Diagnosis
That's the built-in overload protection kicking in. The motor is drawing too much current because the mixture is too thick, the battery is low, or the motor is overheating from consecutive use.
How to Fix It
Change your blending technique. 1) Add more liquid to your recipe. These portable blenders need enough liquid to vortex and circulate the ingredients. 2) Use the pulse function. Hold the button for 2-3 second bursts, releasing between, to chop things up gradually. 3) Make sure the battery is above 25% (more than one solid green light). A weak battery can't sustain the load.
If That Didn't Work
Let it cool down fully. If you've done several blends back-to-back, the internal thermal sensor may have tripped. Set it aside for 20-30 minutes in a cool place, then try a simple water blend to see if it resets.
Heads Up
This auto-shutoff is a safety feature to prevent burning out the motor. Ignoring it and repeatedly triggering it will shorten the blender's lifespan significantly.