Buy a new one and install it. You need something that is at least as powerful as the one you are replacing. And buy a decent name brand supply, don't go cheap your entire computer depends on what power supply you choose. Visit Hardware Revolution for an excellent writeup on choosing a new power supply. Before you take the old one out unplug the machine and press in the power on button for five seconds to drain what you can. Open the case and take pictures of where all the cables from the power supply insert into the mother board and video cards (if any) and sometimes a case fan will be clipped into the power supply as well. Remove the old one after grounding yourself to the case and install the new one. Power supplys are rated in Watts, the one pictured below is 810Watts/67.5Amps. Your supply will have a sticker similar to the one pictured below. Dell use the cheapest and lowest rated supplys they can get away with. Buy something better. If you replace the supply and the computer won't boot either the old supply was good and something else was fried or when it died it took out another component like the motherboard. Check if you have a loose stick of ram, any cable or cord or a card half out of it's slot. Try to reboot again. You may see a steady or blinking amber light on the motherboard that indicates a power problem with the motherboard or power supply. Look for bulged, loose or leaking components on the motherboard. Capacitors especially. Good luck!