Dell desktop suddenly wont power up?

You aren't giving enough info to really tell.. it could honestly be any part bad, the PSU might be unable to power your system, the PSU itself could be broken, the bios could have an issue, and the list goes on. You need to either find someone familiar with building a computer (How to hook **** up) and test each part one at a time on another system, or take it to a computer shop and let them do it.

Edit: To address your additional details.. /repeat... Your computer will not boot if your motherboard will not let it. It could do this for safety reasons because of a faulty part, it could do this from insufficient power, it could do this from too much power, it could do this from a fried bios chip, it could do this from a corrupted bios software, it could do this from the motherboard itself being broken, hell it could do this from a broken wire in an inopportune place. You need to check each part.

If and Only IF you are comfortable opening your computer case, here's what I would try. First, unplug and open the case. Next check the 20 or 24 pin connector to the mother board from the power supply. If this is tight, then chances are you have a bad power supply. Unless it is a mini power supply, you can get a 500watt locally or over the internet for about $25. If you can, borrow a power supply from a friend and try it. If it powers up, then you know you have a bad supply. Other than that, you can unplug all the peripherals even the HD from the power supply and try again. If still no power it is 99% sure it is the power supply.

turn the power switch off for 30 minutes at power supply then turn back on. sometimes PSU lockups can be caused by a power glitch. saw this happen once. we had disconnected all the internal cables at the time but I realized this was not necessary. PSU had its own power switch in back. if it does not, simply unplug the power for 30 minutes and let the power drain from the capacitors.

try unplugging the mouse and keyboard and powering up. do not plug a PS2 mouse or keyboard in while the power is up (damage). you can hot-plug if it's USB.

I have seen a dead mouse prevent a system from booting. dead short of +5V to ground I guess.

if this doesn't work, try a different power supply of the same or somewhat better wattage. PSUs do go out you know. some just last and last.

Probably power supply.

How old is the desktop? Do you have a green light on the motherboard or is it out too?

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EDIT:

I had a very old Dell pentium 4 that I didn't want to put any money in. The power supply was very weak. When the machine had been removed from power for any reason, it was completely dead no light on motherboard-nothing. Oddly, removing the CMOS battery for a minute and putting it back would allow it to boot up. I don't remember if it needed to be unplugged when removing the battery but probably was unplugged. It had nothing to do with CMOS settings but something about removing any residual power. It was a spare machine and I ran it like that on and off for about a year. Only had the problem after a power failure.

Not enough info, but if you've got an amber light, but you can hear your prosessor fan running and the power supply fan running then you could have a bad memory stick or your graphics card has gone bad (or on board video). If your power supply fan isn't running then of course you've got a bad power supply.

Make sure that all physical power connections between the computer and outlet are plugged in firmly.

Make sure that the computer has power going in.

If the computer has power going in and there's no response, I would recommend getting a new PSU (power supply unit) since it might have burned out.

you might need electricity to charge something or so ive heard