This is a known issue that can occur. The following instructions will allow you to restore your page list.
Go to the SQL Console and run the following command
select * from jsonData where XID = 'mangoUI-pages'
Note the ID of the row with XID mangoUI-pages
Go to Administration > System Status > Audit Trail
Filter Type = Json data and find the 2nd most recent record with the id from the SQL Console
Click on the message to see the popup of the Json content. It might not be the 2nd most recent but find the most recent that contains the list of pages in the Json. Copy the Json content and past into a text editor.
You need to remove all the "\" from the content which can be done with find and replace the \ with nothing.
Select and copy just the content between the two [ ]
Go to Administration > JSON Store
Edit the item with XID mangoUI-pages
Past in the pages from the text editor in between the two [ ]
Click SAVE and refresh your pages and your pages should all show up in the Dashboard Designer and Page Editor