In all fairness a lot of people have trouble with the package management system behind a lot of 3rd party Python libraries, it’s still a bit of a mess.
What you’d need to establish is what Python version is running in the environment that Jupyter Notebook is opening with and is there a package named matplotlib in this environment (at present it appears not).
It’s hard to tell without knowing what you used beforehand but you’d probably find it easiest to find out what packages are installed in that environment (depending on your OS this might be a call to pip list or even pip3 list). Make sure you install matplotlib to the correct spot (pip install matplotlib / pip3…).
Edit: A very similar issue that seemed to get resolved recently-
I’d assume that’s running windows so you’ve installed Python yourself (you probably don’t have to worry about pip3). What site-packages do you have installed because it appears that matplotlib is not available. Does pip list indicate Jupyter amongst others but not matplotlib? You’d need to install that package using pip install matplotlib or something along those lines (conda for example typically uses it’s own repos for this package if that’s what you’re using) in order to make use of it.