Have you got any magnetic devices next your PC screen /monitor/? I'm talking about speakers, mobile phones, etc.
The produce magnetic field which interferes the field of the monitor and causes the flickering. Also, have you checked to connections and the VGA/HDMI cables? They may have got damaged.
You can check also check for a resolution to your problem here within this forum: https://ccm.net/faq/6495-monitor-issue-flickering-or-waves As a third option, you can check the advanced display settings. Your monitor might have been set to a non-responsive resolution.