it seems you have plugged in the 8 pin and 4 pin ATX cables.  those are the required connectors to turn on system.  the sata and fans cables do not matter. [IMAGE] [URL] try clear cmos using RTCRST/Password clear/MFG Jumper
Hello everyone!   I am fairly new to the Dell community and computers in general. I have recently received a Dell Optiplex 3040 Mid-Tower and have used it for many techniques. Unfortunately, one day I was messing with it and I purposefully removed the motherboard, power supply, hard drive, and RAM I realized later that what I was trying to transfer it to wasn't compatible, so I attempted to put everything back and screw/plug everything back in place. When I was finished I plugged it in again and it didn't automatically come on. I held down the power button for a few seconds and it still wouldn't turn on. So I opened it again and reconfigured the wires only in vain. I have found many videos about this and attempted to reference it into plugging in the right cables. My suspicion isn't that I broke it, I assume that I'm not plugging in something right. Here is a picture of what it looks like on the inside: [IMAGE][IMAGE][IMAGE][IMAGE][IMAGE][IMAGE][IMAGE][IMAGE] Let me know if there's anything that looks wrong!
Dell Optiplex 3040 Mid-Tower Not Turning On