A login can be locked when:
Its password expires, or
The maximum number of failed login attempts occur, or
The system security officer manually locks it, or
Accounts are locked due to account inacrivity
The system security officer can use sp_locklogin to manually lock or unlock a login. For example:
sp_locklogin "joe" , "lock" sp_locklogin "joe" , "unlock"
Information about the lock status of a login is stored in the status column of syslogins.
See sp_locklogin in Reference Manual: Procedures.