This happens a lot when trying to install Ubuntu Server 14.04 from USB stick (14.04.3 still has the issue).

[!! Detect and mount CD-ROM]
Your installation CD-ROM couldn't be mounted. This probably means that the CD-ROM was not in the drive. If so you can insert it and try again.
Retry mounting the CD-ROM?

.. and of course there is no CD-ROM at all, the installation media are on the USB drive. An easy way to work around this:

1. Open a second console with Alt-F2 (get back to the installation dialogue with Alt-F1 later)
2. Press enter to activate it
3. Enter the following command to make /cdrom a softlink to /media:
   

   rmdir /cdrom; ln -s media /cdrom

4. Go back to the installation process with Alt-F1
5. Answer ‘yes’ to the question Retry mounting the CD-ROM?

From here on the installation should go on as normal.