The Jargon File (version 4.4.7, 29 Dec 2003):

go flatline
 v.

    [from cyberpunk SF, refers to flattening of EEG traces upon brain-death]
    (also adjectival flatlined).

    1. To die, terminate, or fail, esp. irreversibly. In hacker parlance,
    this is used of machines only, human death being considered somewhat too
    serious a matter to employ jargon-jokes about.

    2. To go completely quiescent; said of machines undergoing controlled
    shutdown. ?You can suffer file damage if you shut down Unix but power off
    before the system has gone flatline.?

    3. Of a video tube, to fail by losing vertical scan, so all one sees is a
    bright horizontal line bisecting the screen.


The Free On-line Dictionary of Computing (30 December 2018):

go flatline

   [Cyberpunk SF, refers to flattening of EEG traces upon
   brain-death] also "flatlined". 1. To die, terminate, or
   fail, especially irreversibly.  In hacker parlance, this is
   used of machines only, human death being considered somewhat
   too serious a matter to employ jargon-jokes about.

   2. To go completely quiescent; said of machines undergoing
   controlled shutdown.  "You can suffer file damage if you shut
   down Unix but power off before the system has gone flatline."

   3. Of a video tube, to fail by losing vertical scan, so all
   one sees is a bright horizontal line bisecting the screen.

   [Jargon File]