|Did you remember to plug in the 4 "pin" 12v power connector into the mboard?|
If there is a switch on the power supply itself, are you sure it's in the ON position?
Make sure the recessed slide switch on the back of the power supply is set to the proper voltage - it's often red - it should be all the way to one side or the other.
If you're in Europe or the UK, some cords to the power supply have a fuse or fuses in it, probably at the plug end - make sure it's/they are not blown.
Are you getting something besides no leds lighting up at all?
You may have a poor connection inside your case somewhere.
Unplug the case/power supply.
Power off your monitor.
Open up the case by removing the left panel as seen when you're looking at the front of the case.
Check all the connections of the wiring to make sure they are all the way onto their pins and into their sockets, especially the main connector from the power supply. The wires close to the mboard going into the main power connector/socket should be more or less perpendicular to the mboard surface rather than at an angle. Make sure all cards in slots are all the way down in their slots.
Some Dell cases have a latch you must push one way rather than screws you must remove at the back of the case.
While you're in there, if the cpu fan/heatsink has mung (dust, lint, etc.) on it, clean it off, but DO NOT use a vaccuum cleaner to do that (they produce a tremendous amount of static electricity when running, and anything connected to them can discharge that to your components) - use canned air, or an air nozzle if you have access to an air compressor, or an artist's brush that can be used in small spaces, etc. It may be difficult to clean the top of the heatsink under the cpu fan - the most likely place to have mung on it - and the bottom side of the cpu fan blades unless you remove the fan. If you have a case fan, clean that too if it needs it.
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
"could it be the motherboard?"
If it's the same situation as with the Dimension 2400 series the original power supply may have damaged the mboard while failing. Other than that cause, it's quite rare for mboards to die.
"I thought that even if the motherboard is damaged the cd drive would still go in and out??(which it's not doing)...is this true?"
All computer power supplies are switching power supplies.
They're not like a power transformer. What becomes +12v when it's rectified (turned from AC to DC) is made by rapidly switching the 115v or 230v AC input on and off rapidly - there is no transformer inside the power supply for +12v, but there are small transformers for +5v and +3.3v . An IC chip inside the power supply determines whether it goes fully on or not, and one of the 20 wires in the main power connector on the mboard carries a "power good" signal that is generated by the mboard when you press the power button IF the mboard is working okay. If the mboard doesn't generate the "power good" signal, the power supply cannot turn on fully.
There is no +12v and +5v going to the power connectors to the drives if the power supply cannot turn on fully (hard drives and optical drives require both voltages on a desktop computer. A floppy drive only requires +5v.)
Also, the power supply must have at least a minimal load on it or it won't turn on.