Arduino MEGA r3 power issue

Hi, I have a problem with my Arduino MEGA r3 that I use for a reprap project It's a while since I bought it and it worked fine Now I have a problem powering it When I plug in the Arduino a normal usb cable the board starts correctly and windows recognises it without any issue Then, if I connect the power via the Vin pin (Voltage is about 12.05 Volts), the L led keeps flashing very fast (The arduino has alerady a program but it isn't the blink example) and the board isn't recognised if I plug in the USB cable (a modified one that I modified by cutting the red cable that brings power from the computer) Sometimes the L led blinks at a normal speed but it keeps making 3/2 flashes every 2 seconds I don't know what to do now, because I need the board to make the 3D printer working... Thanks a lot!

Have you tried a normal usb cable? When you power the Mega with the Vin pin, it won't power the usb part of the board.