To reset the administrative password:
su – zimbra
zmprov sp
To get a list of all administrators:
su – zimbra
zmprov gaaa
To access the admin console:
https://YOURHOST:7071
Remember that the administrative console (sometimes) requires a full email address as the login name, so you may be using the correct password and the wrong login!
Resource: https://wiki.zimbra.com/wiki/Admin_Password_Reset