It’s a process of elimination. Unplug external devices like drives see if the PC will boot. Does it make any sound or noise at all. You should hear a fan start up at least. A lot of system will make a quick beep or chirp sound shortly after the power on is pressed. If you hear no noises and don’t see any led lights that indicate power or drive activity, I would say the power supply needs to be replaced. Depending on what you system has for components, you may need a larger supply than the one currently in your system.