my arduino doesn't want to start with the normal usb socket. I wanted to measure how much volts was arriving at the socket, so I accidentally short-circuited the 4 and 5 pin. Then he went on. As far as I know, I short-circuited 5v and a data pin.
does anyone know why the arduino turned on right then?
Your story is not quite clear? If it was not powered, it can't switch on; if it was powered, it was already on. Can you please elaborate a more? What else was connected and how (a simple drawing can clarify a lot)? If it was powered, how was it powered?
This is part of the schematic, are you refering to pins 4 and 5 in the left row or in the right row. Or to other pins like thos on the ICs?