SQL Server restarts, unable to find the reason
One clue would be to look in the system_health extended events session, specifically sql_exit_invoked event.  This will give you the time that SQL Server started to shut down, and you can look in the &quot;shutdown_option&quot; value for additional details.
This will tell you a little bit about how the service was stopped (through T-SQL, or through the services snap-in / configuration manager).
Once you know the when and how, you can dig back in to the SQL Server error log, and / or Windows Event Viewer, from that time period and see if there are more details about what initiated the shutdown.