You are providing very few details for us to help you.
Before “Jupyter suddenly stopped working”, how were you using it? Was it installed on your computer? Using a remote machine via JupyterHub? You tagged this with “JupyterHub” making this more confusing given other things you say. Because later you stress the files exist on your system. A local system or the one running the Hub? Also because you say you tried downgrading to old versions. Both of those latter steps seem to suggest you aren’t using JupyterHub and yet that is the only specifics provided.
If a JupyterHub weren’t involved, I’d just guess it opened your local Jupyter now and you didn’t expect that.
Explore around from inside the active Jupyter ipynb file using ls, ls -lah , & pwd, and then let us know better what you see in text pasted in as formatted blocks. You’ll also need %cd to change directories from within your running Jupyter ipynb file.
Tips for using this and related forums:
Screenshots alone are frowned upon. Please describe things with details in text. And only use screenshots in support of those descriptions when they may not be fully adequate. For example, the 404 image you include could easily be replaced with, “I get a Jupyter branded page saying 404: Not Found. You are requesting a page that does not exist! and the URL in my addess bar is https://xxxxxxx???xxxx.” (Note how that even includes additional information you cut off, which is another reason screenshot are frowned upon as sole details: providers often miss including important details.)
One rationale for screenshots alone not being permitted is because isn’t indexable/searchable by the systems used to organize how to find answers for you and others. Please read Getting good answers to your questions. I’d refer you especially under the section ‘Help others reproduce the problem’, in the linked to How do I ask a good question? at the bottom there where it emphasizes in bold DO NOT post images of code, data, error messages, etc.
In case you didn’t know how to do one step I mention:
For pasting code in so it is readable in forums such as this, you’ll want to learn about ‘block code formatting’ here. (Or see about ‘fenced code blocks’ here. They are both the same thing if you look into the details. They just have slightly different terms used in the two locations.