A lost link is one you had and no longer do. It might have been removed when the post was rewritten, changed to nofollow, or broken because the page moved or your URL changed.
Losing some links over time is normal. It matters most for links you worked to earn, which is why it's worth re-checking won links regularly.
When a valuable link disappears, a short, friendly email asking whether it was intentional often gets it restored, especially if the post was simply updated.
Example
After a blog refreshes its research-apps list, Margin's entry is gone. Margin emails to ask whether it was removed on purpose.
Putting it into practice
Where to use it: Link reclamation email template and Nofollow link checker. For the longer version, read how to monitor backlinks and catch removals.