NOTE: This is for non-media internal links, i.e. links to other non-media item Sitecore pages - Most commonly, when links are broken after migration it is because they are pointing to an old URL which no longer exists. You'll need to clear the link and re-insert it.
1. Determine the link path by opening the page, and hovering over the link in question. The path will appear in the bottom left corner of the window.
If the link path contains "view.asp" - it means that the link is intended to point to a page in Sitecore created during the migration, but the link path has been created incorrectly.
2. The link path should contain the following: a "q=" followed by a string of numbers, for example: "q=266083".
3. Open the page in Sitecore in a new tab, highlight the link text, select 'clear link' and then 'insert link'.
4. When re-inserting the link, type the numbers following "q=" into the search bar in the 'Search' tab - this should return the correct Sitecore page, allowing you to avoid clicking through the content file-tree hunting for the page.