"You can configure SQL Server Agent to restart the SQL Server and SQL Server Agent services automatically if they stop unexpectedly"
Open the SQL Server Management Studio Management folder, right-click the SQL Server Agent entry, and select Properties.
On the General page, select the Auto Restart SQL Server If It Stops Unexpectedly check box.
Here, you should also select the Auto Restart SQL Server Agent If It Stops Unexpectedly check box.
- Click OK.
Read more about Configure SQL Server Agent to Restart Services Automatically
Also check Access control lists for the SQL Agent group in case its related.