Hello,
I accidentally put 12Vdc on the gnd and gnd on the Vin pin.
The board is not broken, the program that is in the arduino stil runs.
But when I put the arduino on the laptop, it does not recognize the arduino.
Its a arduino mega 2560 btw.
What is the fix for this.
I looked at the mega schematic, but the gnd is everywhere.
Does someone knows what component needs to be fixed?
Thanks in advance,
Michel
I would first check out the F1 fuse by the USB socket
F1 is a self resetting fuse.
You may have trashed the regulator and / or the opamp that they use for the power switching. If it was plugged into USB at the time you may have also trashed the 16u2 (does it detect on usb?).
I'm suspecting the 16u2.... Try loop back test
After the loopbacktest the arduino works again? Dont know how, but it does! 
Thank you DrAzzy.
Michel
Because the Polyfuse takes some time - possibly an hour or two - to heal.