If your space's home page has been set to a page that no longer exists (either due to renaming or deletion), then you will need to reset the space home page.
To reset the space home page, click on the Space drop-down menu, and select Space Admin. If you have changed your layout, and no longer see the Space drop-down menu, you can also use the following URL to access the Admin panel of your space:
Once in the Admin panel:
1. Click Edit Space Details
2. Select a new Home Page for your space