My old employee has disabled Windows Authentication in our server. Now I'm not able to access the SQL Server even though I have Administrator access to the server. I need to reset the sa password.

UPDATE

Tried login in Single User mode

But error shows as

Login failed for user 'SERVER\Administrator'. Reason: The account is disabled. (Microsoft SQL Server, Error: 18470)

My old employee has disabled Windows Authentication in our server. Now I'm not able to access the SQL Server even though I have Administrator access to the server. I need to reset the sa password.

I tried logging in using single user mode as Remus described but I get the following error:

Login failed for user 'SERVER\Administrator'.
Reason: The account is disabled.
(Microsoft SQL Server, Error: 18470)