Have a dell optiplex gx520. My little cousins were playing with the keyboard and accidently pulled out the keyboard out of the usb port in the frontal panel and broke the plastic part. So after that I disconnected the plug that goes into the motherboard from the frontal panel. The computer started working. When I turned off the computer to reset the internet it wouldnt turn back on. And I dont hear the fan but there is a yellow steady light on the motherboard. Dont know what to do please help!!

install your chipset drivers

did you change your keyboard and check it out

take a good look inside cause when you removed the front usb you probably removed the front panel buttons too ..Plug them back and will work ..
The fact that it worked is probably why you never seen it ..It does not need the button to be connected in order to work ..the start only kicks it on ..
If this button is damaged ..you can go to bios and look if it has other alternatives fro starting ..(select to start on a set time ..or look if it is capable to start from the keyboard or on mouse double click ..some computers have this option ..for dell i think only to start on a certain hour it is available ..but if you damaged the front button is still an option ..you can also make it sensitive on the ac power ..and if the button is damaged it will start when you plug it in the inlet ..
You may actually try this ..cause unless you plug the button back ..you will not have a way to start it ..so ..try to see if it is set to react on power ..just remove it from the back and place it back ...if you are lucky will start just like it restarts when there is a power fluctuation ..
if not..you need either to reconnect the front panel ..or look on the manual ..and see teh exact pins for the start button ..and short them with a jumper untill it starts ..and make changes in bios ..and use teh keyboard or the mouse or the alarm start until you replace the button or the case...

if u cant install the sound driver just buy a USB sound adaptor and connect it to the USB port and connect the speaker sound jack to the sound port in that....u will have sound....also keep in mind that u " may" have to disable onboard sound in BIOS
get it in Ebay or goole it to locate a place near u

its not the on your USB connection, your keyboard is malfunction... buy a new one or keyboard

Go this mention Site addres & download the drivers http://supportapj.dell.com/support/downloads/ ........................Sameeraryan

have you tried it in a different usb port. or are all the ports not working. Because Dell usb ports have drivers and if the drivers don't work then neither is your keyboard and mouse
Go here and use the 6th one down

http://search.dell.com/results.aspx?s=gen&c=us&l=en&cs=&k=usb+drivers+&cat=sup&x=14&y=9