I reviewed your post and I understand that you are getting check video cable when you turn off your computer.
Don’t worry, I assure you I will try my best to get this sorted.
When you have only the monitor turned on check video cable is the default message and it is normal. As you have mentioned that the monitor is working fine when the computer is powered on I believe that there is no issue with the monitor.
I hope that answers your question and you have a great day!
P.S: Welcome to the HP Support Community 😊
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