A major non-profit organization for a serious illness had a site migration and lost a lot of its traffic.
Unfortunately, we deal with this kind of thing too often.
A shiny new website looks all nice and shiny, but when it's not handled properly in terms of SEO you can lose a lot of your traffic and sometimes quite quickly. Changing a CMS and not having a plan for it can lead to quite disastrous consequences. There are SEO best practices and considerations to take when doing a site migration.
It's always best to talk to a friendly SEO before you press the launch button, and even better to engage with an SEO team at the outset of any website design project.
Fortunately here we were able to go back using analytics and log files and reconstruct the URL paths and put in appropriate search engine-friendly redirects. You can see the recovery to pre-site migration levels.