Another reason for SQL Server Agent issues is if you are running SQL Express. SQL Express doesn't support SQL Agent.
But, as John Mitchell suggests, check the logs. Also, check the service to make sure it is running as an account that is currently enabled and the password is correct. I've seen (in very rare cases) where the service was set to start as the user who installed the instance. This was due to a mistake that the user made while installing the instance but it worked at first... until he reset his password. Then after a reboot, the SQL Agent failed to start.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.