User accounts are subject to the security settings that have been defined in Group Policy. One of the most common settings related to user account password is that the account should be locked out after predefined failed login attempts. This occurs when a user has forgotten her password and makes several consecutive attempts, guessing wrong each time or another user try to hack the user password by trying different sets of passwords.
To unlock a locked account, open the Active Directory Users and Computers MMC snap-in, right click the user object and select Properties from the context menu. In the user Properties dialog box, select the Account tab and uncheck the Account Is Locked Out check box.
(To open Active Directory Users and Computers MMC snap-in (DSA.MSC), select Start > Administrative Tools > Active Directory Users and Computers or type DSA.MSC in the run dialog box and hit enter)