Sitecore Tackled

Has your IndexWorker service stopped?
To fix this we need to start the service and rebuild the _xdb index in Solr.

This is pretty straight forward and explained well in Sitecores documentation here: https://doc.sitecore.com/developers/91/sitecore-experience-platform/en/rebuild-the-xdb-index-in-solr.html

1. You need two cores in Solr (read the sitecore doc I linked for this if you need to create the _rebuild core).
   – <name>_xdb
   – <name>_xdb_rebuild
   
2. Next, in powershell, find your IndexWorker instance folder. i.e. on Sitecore 9.1.1 XP0 it’s:<path>\site.xconnect\App_Data\jobs\continuous\IndexWorker.
   
3. Run .\XConnectSearchIndexer -requestrebuild

This will rebuild the index and swap the Cores with eachother. After it’s done you should see that your data pops up again in Xdb.
Good luck!