Most common causes of problem:

Motherboard, Power Supply, Power Cable/Outlet.

Solutions:

Check the power supply switch is turned on.

Try another power cable/power outlet.

Check for loose connectors inside, try unplug and re-plug internal connectors such as CPU power, motherboard power, and front panel power/reset headers.

Replace the motherboard/power supply.

Most common causes of problem:

Motherboard, Power Supply, RAM

Solutions:

Try another power cable/power outlet.

Check the motherboard/CPU power cables are plugged in properly.

Check the RAM is installed properly/not loose. Take it out and put it back in/change what slots they are installed in.

Make sure the RAM is a compatible model.

Check the CPU/GPU/motherboard temperatures to make sure it's not overheating.

Replace the motherboard/RAM/power supply.

Most common causes of problem:

Motherboard, RAM, graphics card.

Solutions:

Make sure the display output cables are plugged into the graphics card instead of the motherboard if the PC has a discrete graphics card.

Check the power connectors to the graphics card are plugged in properly.

Check the RAM is installed properly/not loose. Take it out and put it back in/change what slots they are installed in.

Make sure the RAM is a compatible model.

Replace the motherboard/RAM/graphics card.

Most common causes of problem:

Motherboard, SSD/HDD, RAM

Solutions:

Check if the SSD/HDD is detected by the BIOS, and if it's installed in a compatible slot (not a M.2 SATA drive in a slot that only works with M.2 NVMe drives, or A 2.5 SATA plugged into a port that gets disabled when multiple drives are installed, etc...).

Reinstall Windows (via USB).

Check disk health for the SSD/HDD to see if it's damaged/dying.

Check the RAM is installed properly/not loose. Take it out and put it back in/change what slots they are installed in.

Make sure the RAM is a compatible model and running at supported speeds/latencies (try disable XMP/EXPO).

Replace the motherboard/RAM/SSD/HDD