It could be that the installer re-created the admin account, or reset the password - I'll check that out.
In the meantime, tho - does the account still exist? (zmprov gaa)
If so
zmprov sp <account> <new password>
will reset the password.
Creating accounts:
zmprov ca <account> <passwd>
Admin accounts:
zmprov ca <acccount> <passwd> zimbraIsAdminAccount TRUE
Turning an existing account into an admin account:
zmprov ma <account> zimbraIsAdminAccount TRUE
Downgrading an admin account to a normal account:
zmprov ma <account> zimbraIsAdminAccount FALSE