Sometimes you can get a notification from us that you lost a link. Here are some fast examples and what can you do:
If the server down, you can wait few days to see if it comes back up.
The backlink not found can be either by someone removing your link, or the actual link was on that page (homepage) and now it is in an article, and you have to update that link.
On 404 Not found, you must check why that page is gone (either by changing its structure or other reason).
In some cases you can’t do anything, but it’s good to know if you lose a good link that can affect your SEO rankings so you know you must work to get other good links.