Have you ever changed the logon password for your SQL service account and found that the service no longer starts up after a reboot on it's own? You look through the event log and you see the error:
The service did not start due to a logon failure.
The MSSQLSERVER service failed to start due to the following error:
The account name is invalid or does not exist, or the password is invalid for the account name specified.
But when you go into Services, re-enter the login info, and restart the service, it comes up just fine. Reboot and the problem comes right back.
I've seen this issue and dealt with it for years now, since the release of 2K, probably. It's really annoying because you have to manually restart the service every time you reboot. And of course the change never sticks. Guess what. In true Microsoft fashion, they put the login info elsewhere in the OS, and in this case, it overrides the Services tab login info.
To get the login info to stick, you have to right-click on the server instance in Enterprise Manager, select Properties, enter the security tab, and re-enter the account password here. This setting apparently overwrites the Services setting upon each reboot.
Thank God I won't have to babysit that server any more. All I have to says is: FINALLY!