To restore a mind map that was deleted by mistake please check your trashcan. See the screenshot below.
If there are topics in your mind map that got deleted by mistake use the keyboard shortcut ⌘Ctrl+Z to undo those changes or use History View to see the full list of changes for your mind map.
Tip: If your mind maps are missing and you can't find them in the trashcan, please make sure you're not logged in with a different email then the one you used when you created the mind maps. It's the most common cause for missing maps.