This is not pertinent to this forum. If in the same environment where your kernel is running, you ran this code in a Python console or in a script that was called from the command line, you’d likely have the same issue. That eliminates Jupyter as the causing factor. (This is a thought experiment to always consider when deciding where to seek help.)
I know things have changed and maybe that is the case and you just need to adjust your code. This issue posted in the qiskit issue dashboard 2 weeks ago shows that Aer is now imported from qiskit_aer. That also involves QuantumCircuit and so I think that is hinting at the direction for updating your code.
from qiskit import QuantumCircuit
from qiskit_aer import Aer