Upgrading of Ubuntu 14.04 to Ubuntu 16.04 fails

While upgrading Ubuntu 14.04 to Ubuntu 16.04, I have received the following message:
"After updating your package information, the essential package
'ubuntu-minimal' could not be located. This may be because you have
no official mirrors listed in your software sources, or because of
excessive load on the mirror you are using. See /etc/apt/sources.list
for the current list of configured software sources.
In the case of an overloaded mirror, you may want to try the upgrade
again later.

Restoring original system state"
Will you kindly suggest what to do next for a successful update?