This morning our DBA was unable to get into SSMS, upon checking the logs, we saw this
FCB::Open failed: Could not open file D:\Database\MSSQL11.MSSQLSERVER\MSSQL\DATA\mastlog.ldf for file number 2. OS error: 5(Access is denied.).
The MSSQLSERVER service wouldn't start. The logon service was NT Service\MSSQLSERVER. This was the default, and never was changed.
The SQL server is on a domain. For now we added modified the permissions and added NT Service\MSSQLSERVER to Modify on the folder where the ldf file was located. This fixed it.
My question is
Why did this suddenly occur? The user has been using the server for many months like this.