Any idea what model Dell it is?
A majority of dell desktops have a green LED on the motherboard to indicate it’s ready to power on.
Things I would suggest trying
-Make sure the P4 connector is attached properly (if applicable)
-Try shorting the jumper for the power switch to see if it powers on
-Clear the CMOS by pulling the battery & power
-Reseat the CPU
Usually the most Dell’s are properitary in the sense that you can’t bolt a power supply to the case. The connectors are almost always wired in the conventional way.