This can happen if you deleted your browser cookies for datascientistworkbench, or if you launched Jupyter in a new Private (Firefox) / Incognito (Chrome) window, or due to a bug.
Don't worry, your notebooks (and data) are still available in Data Scientist Workbench, and you can get to them using one of the following ways:
1. Click on the See all notebooks link under Recent Notebooks in the right nav bar.
2. Use My Data: Just click the My Data (folder icon) in the left menu bar.
Once it opens just click Back, to go up one level to /resources and you can find your Jupyter notebooks there (may need to scroll down a little).
Click on the desired notebook (.ipynb) file and it will open up in Jupyter.