When you delete an abbreviation set, Typinator first warns you. If you confirm the warning, Typinator does not really delete the set, but rather moves it into the Trash. If you have not emptied the trash yet, you still may find the deleted set there. To recover the set, move it out of the trash and double-click it to re-import it.
If the deleted set is no longer in the trash, you can recover it from a backup. If you are using Time Machine, you first need to find the folder where Typinator keeps the sets. Open Typinator's preferences and switch to the Expansion tab. In the "Sets folder" section, you can see the folder's location on your computer. Click the "Open in Finder" button to quickly navigate to the Sets folder. Then click "OK" to close the preferences window. In the Finder, enter Time Machine and go back in time until you find the lost set. After restoring the set, it automatically appears in Typinator's set list.