Please read Getting good answers to your questions and revise your post accordingly.
There’s a lot of ways to use Jupyter and you detail nothing about how you are. You may have simply lost the internet connection if it was remote.
Did you try a different browser?
Restart your machine?
Even the tags you added are confusing. Is it JupyterLab with the problem or Jupyter Notebook or both?
Where were you working on this project? If it is associated with an institution, they may offer more help than we can.
Finally, what kernel were you using? If it was Pyodide or Xeus-Python in JupyterLite (like you get if you try the first two tiles here that are prefaced as being ‘Experimental’), that is more limited.
In the meantime, you can use Jupyter remotely a couple of ways. One is by clicking the ‘launch binder’ badge here to get temporary ipkernel session. Or try Anaconda Cloud. Both require you to install things if you need more than basics, but they may help you troubleshoot.