Fix "SQL SERVER AGENT START ACCESS DENIED" 

See the logs for the exact issue:
1.Provide the permission on the sql server agent folder.
2.Add the sql server agent service account as login with sysadmin. Keep this as a Optional
3.Execute the below commands
USE master;
GO
EXEC sp_configure 'show advanced option', '1'; --Enable advanced options
RECONFIGURE;
EXEC SP_CONFIGURE 'Agent XPs', 1;
GO
RECONFIGURE;
GO
4.start the agent if it is not restarting talk to client for downtime and restart the sql server and start the agent manually using services.msc if not started.
Please do comment if any steps helped you in your real time.