Ubuntu screen appearing after several hours of PC being idle

[wolfman]

I would try deactivating the screen saver lock and computer idle buttons and see if that is causing it?.

What GPU drivers (if any) do you have installed?:

How to install graphics card drivers in Ubuntu

ATTENTION ATI/AMD GRAPHICS CARD OWNERS, IF YOU INTEND TO USE UBUNTU (MATE) 16.04, PLEASE READ THE FOLLOWING FIRST!. EDIT: POSSIBLE SOLUTION FOR SOME WHO HAVE AN AMD CARD ON THE LIST HERE: http://support.amd.com/en-us/kb-articles/Pages/AMD-Radeon-GPU-PRO-Linux-Beta-Driver–Release-Notes.aspx See also: https://support.amd.com/en-us/download/linux First and foremost, Intel drivers are auto installed and there is no reason to start to try installing any drivers for Intel graphics cards as the …

Also, try changing your software sources download location and updating again:

Ubuntu Mate Update Guide

These are my own setting preferences, use at your own risk!. AN IMPORTANT NOTE FOR LINUX BEGINNERS. WHEN YOU ARE IN RECOVERY MODE, THE KEYBOARD DEFAULTS TO US ENGLISH, IF YOU HAVE ANY OTHER LANGUAGE KEYBOARD OTHER THAN US ENGLISH, WHEN YOU TYPE "Y" (FOR "YES"), IT MIGHT SHOW UP AS A DIFFERENT LETTER, ON MY KEYBOARD I MUST PRESS THE "Z" KEY WHICH EQUATES TO "Y" AS I HAVE A GERMAN KEYBOARD, JUST PRESS "BACKSPACE" AND HIT A DIFFERENT KEY UNTIL YOU SEE THE "Y" SHOW UP!. Click on "Subscribe…