Does SQL server 2008 log instance level security permission changes
If I'm not mistaken the default trace contains property changes to logins which should include an account being disabled. An article that walks though the various information you can get from the default trace.

EDIT

OK, already confirmed not much at all is captured with manipulating logins on an instance. So unless you had something already capturing the information like SQL Server Audit (if on 2008 or higher), there is nothing natively logged by SQL Server to let you know who did it. 

If you happen to be logging successful logins to that instance you can determine who was logged in that had the permissions but that is the closest you will likely get.