First of all, the "Editor -> Swiftify" menu item will only appear when you have a file opened in the Xcode editor.
If the "Editor -> Swiftify" menu has disappeared from Xcode, the first thing to look at is whether "Swiftify" is checked in the System Preferences -> Extensions dialog:
If the "Swiftify" extension is not listed, move "Swiftify for Xcode" from the Applications folder to Trash and reinstall "Swiftify for Xcode" from the AppStore.
If the "Xcode Source Editor" item is completely missing on the left, this is most likely due to a corrupt installation of the Xcode itself. The best suggestion here is to move "Xcode" from the Applications folder to Trash and reinstall Xcode from the AppStore.