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I would suggest checking if your HDMI cords are firmly connected. You can also try connecting to an alternate HDMI port on your monitor or television or using an alternate HDMI cord
PS4 Safe Mode Options 4 or 5 may help as well
If that doen't resolve issues you can try reinstalling the system software with steps 6 or 7, but you should back up your saves first to USB or Online Storage.
Where do I find the ps4 safe options?
Here are some instructions for PS4 Safe Mode:
You should back up your saves to online storage to to USB before doing anything that initializes your PS4
For back up options go here