Hi @mmhasan,
I hope you enjoy your visit to the HP Support Forums. I understand you are having an issue with message you are lockout. I will be glad to help you.
Per the User Guide page 8, If pressing the power button has no effect, the power button lockout feature may be enabled. To disable this feature, press and hold the monitor power button for 10 seconds.
Please keep me informed of how things went.
If you require further assistance, let me know and I will gladly do all I can to help you.
Please let me know if this resolves the issue by marking this post as "Accept as Solution".
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Thanks.