IF you're seeing this message on the monitor, it could mean several things.
First, the cable connections could have come loose. So check the connections both on the PC and on the monitor.
Second, if those are good, it could mean the video cable has frayed or otherwise gone bad. You would need to replace it with another cable.
Third, if you are using a known working cable, then it could mean that the video chip in the PC has gone bad, or if you are using an add=on video card, that has gone bad.
Unfortunately, with no working video, you are going to be unable to run diagnostics to see anything. So basically, you would need to take this to a local PC repair place to see what they can do about checking out the video issue.
IF it is still under the original HP 12-month warranty, then you can contact HP Customer Support to have them put you in touch with HP Technical Support. To contact HP Support Goto this page https://support.hp.com/us-en/contact-hp
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