This article applies to the situation where Google Analytics user and event data in the past, usually after 26 months, disappears or is no longer attainable in the results.
This happens due to Google changing their user data retention policies for GDPR to be a default of 26 months. Anything older than that point is automatically deleted from their system, unless the administrator manually changed the retention period for the web property.
If this occurred, you won't be able to recover the data. But you should be able to follow the steps in this guide to update your user retention policy period to be longer so more data is not lost.