Sql-server – SQL Server Agent refuses to start

I'm having a problem with SQL Server 2005 where the SQL Server Agent suddenly refuses to start. If I attempt to start it through Services, I get the error "SQL Server Agent (MSSQLSERVER) service on Local Computer started and then stopped." In the Application log, I have the following entry:

Event Type: Error
Event Source:   SQLSERVERAGENT
Event Category: Service Control 
Event ID:            103
Date:       5/20/2010
Time:       11:07:07 AM
User:       N/A
Computer:            SHAREPOINT
Description:
SQLServerAgent could not be started (reason: Unable to connect to server 'SHAREPOINT'; SQLServerAgent cannot start).

This database has been running fine for four months. It contains a SharePoint configuration database, which two days ago stopped working, throwing me a message that the configuration database cannot be reached. It was then that I realized the SQL Server Agent was not running, and I have been unable to restart it. I have tried running it with both the local system account and the network service account, with the same results. So far, I have tried:

• Granting the administrators group, network service, and SharePoint SQL Server Agent account public and sysadmin roles on the database.
• Granting the administrators group, network service, and SharePoint SQL Server Agent account full permissions to the entire MSSQL directory and all files within.

I still have no joy.