It hit its auto-stop timer. The K400 stops automatically after 3 minutes on variable speeds (1-5).
- Press the stop button.
- Unplug the blender.
- Check the control dial. If it's on a preset recipe program, it will also auto-stop when the cycle finishes.
If that didn't work, the jar might not be properly aligned on the base. Unplug it. Remove the jar. Place and align the jar with the slot in the jar pad, facing the jar handle towards the control dial. Plug it back in and try again.
Heads up: Always unplug the unit before removing the jar or lid. Allow the blender to fully stop before pouring.