I'm doing a project using mega 2560 and it takes vin from external 11.1v battery connected to a switch,usually i connect my arduino to my laptop while power is on and no problems happen, then i shorted 5v and gnd by accident, i tested the arduino by uploading blink and other sketches, testing differnet pins and trying to connect sensors to the arduino's 5v, it worked fine. i was trying sth while connected to my laptop and i wanted to reset the arduino " im using a shield so cant use reset button, i turned the on/off switch bec i forgot it was connected my laptop force shut down in less than a second, basically when i connect usb to laptop and open external power laptop force shut down and bios settings like date and time are reset, is ther a problem with the arduino or my laptop??
That sounds like the laptop is seeing something on the USB port that causes it to panic and shut down, presumably either that 5v is shorted and the laptop's motherboard is of shoddy design and powers down entirely instead of just shutting off the port, or that the voltage on the pin is significantly higher than 5v, polarity is reversed, or something else really bad like that.
Sounds like the arduino's power supply circuitry was damaged, and the 5v from arduino's regulator is feeding back to the USB 5v. This shouldn't happen, of course - that said, I have a very low opinion of the power switching circuit on the full-sized boards, and it seems prone to bad behavior. On the third hand, I am also suspicious of your project wiring as well as the laptop itself.
Does the board otherwise still work? I can definitely think of ways that one could miswire things so that you trashed the board and it would cause the laptop to panic when plugged in leading to a shutdown as you described, but I am having a hard time thinking of cases where it would do that without also destroying the arduino. Assuming it does, what about
Definitely avoid doing what you did to cause this failure initially as it is clear that the laptop is detecting conditions that could be damaging to the laptop - until you understand the problem and have corrected whatever led to it.
If it still works, what voltage do you measure with your DMM between Gnd and 5v pins running off external power?
You didn't by any chance have the power supply connected to external devices, but the connection to the arduino only turned off... if the stuff you have connected was powered, and might have been supplying a voltage on other pins to the arduino, that could cause problems. Disconnecting power from the ardino (even if it is connected to USB; This is supposed to be okay, but it is the sort of thing I would not ever want to do to my hardware, especially when it's connected to the USB port of my computer) while any of it's pins are connected to something with an external voltage on it, that is bad.