There are two causes for the website to be reset:
[REASON 1] You accidentally pressed "Reset" or "Change template" button. Check if you have manual or automatic backup enabled. Hover on "Publish" icon and you will see "Backup / Restore" item;
[REASON 2] You closed the website builder without pressing "Save draft" after creating a website. Always make sure you saved your website before closing the browser tab. We also advise to make website backups regularly.