Computer won’t start but has power

My computer has power but won’t boot. I also connected my hard drive to other computer and it’s booting the computer. I replaced also my power supply unit but still won’t boot. it still response to F2 button to go to the BIOS and I have check my hard drive still readable. But after rebooting the computer it won’t boot even the RAM reading or the initialization don’t show up.
What device could be the problem?
