Greetings,
Welcome to the forum.
I am not a HP employee.
There is no quick solution to this symptom.
A number of things can cause this; you have to troubleshoot components and connections to find the cause. You may need replacement components and will have to reduce the PC to a minimum configuration by removing components, one at a time, swapping in replacement components, and test booting to see if the PC will POST.
Typical causes:
1. Power supply
2. Power switch
3. Motherboard
4. Component connected to the motherboard directly or via cabling.
5. Peripheral device connected to the PC.
You may consider contacting HP Support (Link) if in warranty or have a local PC technician take a look.
Regards