I had faced the same problem. Restarting the notebook solved my problem.
If that doesn’t solve the problem, you can try this
pip install seaborn
Edit
As few people have posted in the comments, you can also use
python -m pip install seaborn
Plus, as per https://bugs.python.org/issue22295 it is a better way because in this case, you can specify which version of python (python3 or python2) to use for running pip