When your service account is expired/disabled or is not having access than sql service could not able to start and you may get an error an en eventvwr as
“Could not open error log file ‘C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\LOG\ERRORLOG’. Operating system error = 5(Access is denied.).
so you just change the service account and it should work, generally you change the service account using services.msc which is not a good way to change the service account, and could not effectively change it, and error still could persist.
So recommended way of changing the service account is use “Configuration Manager” which takes effect efficiently.
Lesson Learnt “Always use Configuration Manager for changing service account”
Ref: Practical Troubleshooting Sql Server – Ken