Ubuntu 15.04 has reached the end of life. Here the announcement:

This is a follow-up to the End of Life warning sent last month to
confirm that as of today (February 4, 2016), Ubuntu 15.04 is no longer
supported. No more package updates will be accepted to 15.04, and
it will be archived to old-releases.ubuntu.com in the coming weeks.

The original End of Life warning follows, with upgrade instructions:

Ubuntu announced its 15.04 (Vivid Vervet) release almost 9 months
ago, on April 23, 2015. As a non-LTS release, 15.04 has a 9-month
month support cycle and, as such, the support period is now nearing
its end and Ubuntu 15.04 will reach end of life on Thursday, February
4th. At that time, Ubuntu Security Notices will no longer include
information or updated packages for Ubuntu 15.04.

The supported upgrade path from Ubuntu 15.04 is via Ubuntu 15.10.
Instructions and caveats for the upgrade may be found at:

https://help.ubuntu.com/community/WilyUpgrades

Ubuntu 15.10 continues to be actively supported with security updates
and select high-impact bug fixes. Announcements of security updates
for Ubuntu releases are sent to the ubuntu-security-announce mailing
list, information about which may be found at:

https://lists.ubuntu.com/mailman/listinfo/ubuntu-security-announce

Since its launch in October 2004 Ubuntu has become one of the most
highly regarded Linux distributions with millions of users in homes,
schools, businesses and governments around the world. Ubuntu is Open
Source software, costs nothing to download, and users are free to
customise or alter their software in order to meet their needs.

On behalf of the Ubuntu Release Team,

Adam Conrad

  Ubuntu 15.04 (Vivid Vervet) End of Life reached on February 4 2016