Greetings Lulu1018,
Welcome to the forum.
I am not a HP employee.
Please provide additional background information which may help to diagnose your problem. Please provide your PC's product number only. This information will help to check the age of your PC.
You can request HP warranty service (Link) if your PC is in warranty. PC failure occurs with greater frequency as time goes by.
What do you hear and see when you power on your system? Any HDD or DVD initialization sounds? Any LED activity? Do you see any screen activity when you power on the system?
Check AC power connections: to the wall, the external power supply, and the PC.
I wish I could provide more feedback.
Possible causes of this symptom are many. Replaceable hardware failure (HDD, RAM, or monitor failure) to complete motherboard failure or external power supply failure can cause this symptom.
Regards