Why my pc is not working

So, I have a Dell Optiplex 4060, I think, and it has a processor which is an i3-6100T, and it has 4GB of RAM, but suddenly today, even though it was working just yesterday, when I clicked the power button, I saw it like turn on, the LEDs turned on, and the fans turned on, and it just turned off after it, just turned off, and when I tried to turn it back on, it won't turn on, even though it was working fine just yesterday, and I tried shorting the pins, reinstalling the RAM, and like removing everything unnecessary to boot, and also replacing the CMOS battery, and when I turned it on yesterday, it has like a time error or something, I don't know, and I guess that was because of the CMOS battery being dead, yeah, and it's not really necessary to boot, so I didn't really care about it, and I tried today to replace the RAM place, and just replace the CMOS battery, and my power supply has a button on the back, which when I click it, the fans turn on, I don't know what it is, I think it's just like for testing the power supply, so I know now that the power supply is working, and also the motherboard, because the power needs to be passed from the motherboard to the fans, so the motherboard is good, and I don't think the CPU can be faulty, because I never touched it, so what can be the problem?