Desktop PC not powering up

Try swapping out the power supply with one of your other computers'. If it boots you know you just need a new power supply.

You don't specify how old or what model the Dell is. If it is an older system, Dell used to use proprietary power supplies. If this is so, you can only get a replacement from Dell, though one from one of the other Dells that is older as well may work.

Again, make sure the power supply will be compatible before swapping it out.

Also check and see if any cables are loose. If they are, some motherboards / Power Supply Units have a safety that just doesn't boot the system if anything is loose.