iMac turned on after repairing, but didn’t start again after that

My iMac (late 2014, 27 inch) stopped working so, I opened it up and figured out that the problem was in the power supply. I replaced the power supply and started it again, 3 LEDs were on (first 3 from bottom, I hadn’t connected the display) the iMac chime played but then I was unable to turn it off via the power button, so I pulled the plug. Then I tried to start it back up again but only the bottom most LED was on and the power button wasn’t doing anything. What could be the problem.