Have a dell optiplex gx520. Little cousins broke the plastic part of the USB port in the front panel. So I disconnected the the front panel connector and then the computer turned on. It was turned off about an hour later and now I cant seem to turn it back on. I dont hear the fans or anything. Please help. Thanks.

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...