You likely disengaged the coupler by carrying the blender incorrectly.
Do not carry the blender by the jar or jar handle. Always lift and support the unit from the blender base. The base will detach from the jar if you lift it by the jar alone.
To re-seat it: Unplug the blender. Place the jar on a flat surface. Align the jar with the slot on the blender base, ensuring the jar handle faces the control dial. Press down firmly until it clicks into place.
If that didn't work: The coupler teeth may have come off. This can also happen if you push down on the lid or remove the jar while the blades are spinning. The unit will need service if the connection remains unstable.