Press ⌘Z (Mac) or Ctrl+Z (Windows) to undo the last delete or edit; ⌘⇧Z or Ctrl+Y redoes it. A deleted line also shows an Undo button for a few moments.
Inside a cell, undo works on the cell until you start typing; after that it is the browser's own undo for the text.
To go back further, open Version history and restore a snapshot. A restore point of the current state is saved first, so nothing is lost.
Not what you were looking for? Write to hello@actualsbudget.com.