Unable to unmask, start or enable a service because the unit is masked

Issue

• After masking a service, it is not possible to unmask, start or enable it again.

• Getting messages as below in the system log:

  systemd[1]: Cannot add dependency job for unit sshd.service, ignoring: Unit is masked.
  

• Getting messages as below while attempting, start or enable a service:

  # systemctl start httpd.service 
  Failed to start httpd.service: Unit httpd.service is masked.
  # systemctl enable httpd.service 
  Failed to enable unit: Unit file /usr/lib/systemd/system/httpd.service is masked.
  # systemctl enable vncserver@:1.service
  Failed to enable unit: Unit file /usr/lib/systemd/system/vncserver@.service is masked.