The Search Engine Metainfo Dashboard is a table that allows you to set and override with custom and specific metainfo each links contained in the sitemap of your site.
Each record is associated uniquely to a link URL, so if you have saved a metainfo title/description and later on it's all gone, this means that URLs have been changed someway.
Pay attention to not login to the backend using a different domain
There are no other reasons why a metainfo record may disappear, data are stored in the database so unless the database table 'xxx_jmap_metainfo' has been deleted it means that URLs are not the same you saved data for.
If you previously entered some values for the Metainfo records and now they don't show up anymore, it probably means that you are logging in backend with a different domain/www prefix.
Metainfo are associated to each link, if links change metainfo change too.
If you specify metainfo for http://www.mysite.com/ and open http://mysite.com/ without www prefix they won't apply, it's a different link. Most importantly, if your website is accessible with doubled domains http://www.mysite.com/ and http://mysite.com/ stop to do this immediately. It's a very bad practice for SEO, so consider to setup a htaccess rewrite/redirect rule to the preferred domain matching the one set in your Google Search Console.
The same happens if you had links without SEF rewriting http://www.mysite.com/index.php/mypage.html and change them to http://www.mysite.com/mypage.html activating the Joomla SEF rewriting.
The same happens if you change links for https starting with http://www.mysite.com/mypage.html and change them to https://www.mysite.com/mypage.html
Every time a link changes the meta data associations is lost. Keep in mind that the URL is the unique identifier for meta data that you entered.
If you are migrating a website from one domain to another, refer to this FAQ for Metainfo migration strategy: