I have deleted something but I had copied before. Ctrl+v shows only the last copied text. I tried to install diodon and cliplt. As I understand they start to archive some history of the clipboard. But how can I go to some history of the clipboard within Ubuntu 14.04? Leaving my PC turned on while waiting for answers.
If you don't have a clipboard manager then it's gone forever.
If you are using KDE then you are in luck because it has all the clipboard history under the scissors icon in the tray
To install one:
sudo apt-get install parcellite
It runs in the Notification Area and offers many options for managing both Primary Selection and standard freedesktop.org Clipboard.
As the previous answer states: If you don't have a clipboard manager then it's gone forever, a good alternative to Parcellite is CopyQ, an advanced multiplatform clipboard manager with editing and scripting features.
It is included in Ubuntu 18 bionic. For older other releases execute the following in a terminal (Alt + Ctrl + T):
sudo add-apt-repository ppa:hluk/copyq sudo apt update sudo apt install copyq
Detailed info: https://hluk.github.io/CopyQ/