Hi, are you running the most recent version? We had some issues at the beginning of the year where you could end up with more than one editor for the same project open in the browser. This meant that when you closed the editor that you had made changes in and then later closed the other version of the editor, the data was overridden by an old version. I think this was fixed in v3.2 so you should no longer be able to have multiple editors open within the same browser.
If you look in the user-files folder for the project on the server you might be able to get the lost data back. Preview xml files are saved when changes are made in the editor and data.xml is saved when the project is published. If it was caused by the browser issue above and you haven't published the project since noticing then data.xml may still contain the data you need. If it does then save it as preview.xml, open the editor and you should see the lost data. Otherwise you will have to look through the other preview & data files to see if they contain the missing data (the last 10 versions of them are saved). Backup all the project's user-files folder before you start renaming things.
I hope this helps