Ubuntu Linux by default uses a windowing framework called X to provide display support for a wide variety of hardware platforms. The X window system, like its built-in counterparts in Windows and Mac operating systems, provides a common interface between the operating system and the hardware for different monitors attached to the computer. It uses standardized display connection protocols, such as HDMI, VGA or DVI. Hardware and software connectivity and compatibility problems can cause the operating system and by extension the windowing system to fail to detect a screen connected to the device.
There a number of problems that can contribute to the Ubuntu "No Screens Found" error on boot. Hardware connectivity problems, such as loose connectors, bad cables or damaged plugs contribute the most to screen detection failure in Ubuntu. This is especially true with VGA and DVI connectors, whose pins are prone to bending and breaking if the connector is not carefully aligned on plug-in and removal. Software problems include auto-detection failure or misconfiguration, and out of bound values in driver configuration files for X11, which will result in an Ubuntu system error. In the case of a "No Screens Found" error, the system may be able to display textual messages during boot but may fail to display other graphical windows.
It is crucial to ensure hardware is functional and connectors, cables and adapters are in good condition before installing or changing system configuration on Ubuntu. The Ubuntu auto-installer and screen detection system detects the vast majority of properly functional displays connected to the computer, but cannot be successful if there are connectivity problems. Always backup the X11 display configuration file if editing the driver configuration manually. Erroneous values in the Xorg configuration file yield "No Screens Found" or similar errors in all versions of Linux that use X11 as a windowing system.
Ubuntu "No Screens Found" errors appear on boot, before X11 completes loading. If a "No Screens Found" error is displayed in text on the screen, the computer will stop the boot process but will not shut down. To diagnose the error, try shutting the computer down by holding the power button, then verify that all hardware cables and pins are connected and intact. If, upon restarting, the error still appears, there is likely a problem with the Xorg configuration files that control the monitor drivers. These can be verified in the system log files stored typically in "/var/log/Xorg.0.log".
Configuration errors resulting in the "No Screens Found" message can be reset by replacing the Xorg configuration file with a saved backup or by running the X configuration tool, packaged by default on all Ubuntu Linux distributions. From a command-prompt, the command "X -configure" will run a wizard to detect and configure to system defaults all attached screens. After successful reconfiguration and reboot, the system should be able to use the monitors correctly and the error message will disappear.