iMac won’t turn on after hard drive replacement

I attempted to upgrade its hard drive on my own but in the process something happened, I don’t quite know what, and now it won’t turn on. When it’s plugged to the power the internal green leds are on and when I press the power button the rest of these leds turn on for a moment and the fans attempt to spin but then nothing happens. I check the connections and everything I think of and they seem good.  Yet the Mac won’t turn on.

Help please!