You might have deleted a video by accident, but no worries! All deleted videos are archived in your personal trash can.
You can find this section at the bottom of your My Moovs page (the section will only be visible if you deleted at least one video): click on the little arrow next to the "Trash" title to view all deleted videos. Click on the "Restore" icon in the bottom right corner of the video thumbnail to recover a deleted video. It will then be moved back to your project list.
If you wish to permanently delete the archived videos, click on the "empty trash" button at the bottom of the page.