|Sometimes things can get damaged if you don't unplug the AC power before digging around inside the case since most computer power supplies provide a small current to the motherboard even when the computer is shut off.|
Or you didn't get everything back together correctly. Or maybe you had some static on you and it discharged into some of the hardware while you were inside the case.
Hard to pinpoint for sure. There's a million ways for a computer not to work and only one way for it to work.
But you can try temporarily disconnecting all the drives and removing any unnecessary card and then see if it'll at least show a posting screen or Dell logo. when you turn it on. If so, then one of the disconnected items must be at fault.
You're not really green until you're soylent green.