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?