Unfortunately there is no protection on the mega for someone applying 5v to VCC and plugging in usb 5v (there is if you use vin and usb, it safely selects Vin over usb)
So the two 5v's will battle it out since they are most likely not identical 5v. till one of them wins...
I would guess you have fried your usb port power...
Have you got a usb power meter ? check its still putting out 5v ?
Also a lot of USB chips have protection in them, but you have to completely power off for a while (including removing batteries in laptops)
Try a different usb port.
If it still doent work... mega has an issue
5v on usb goes through a fuse then a mosfet then into the atmel 2560 mega. With a multi meter check 5v is on a 5v pin of the mega when plugged into usb.
if its not there check the fuse (pictured below)
If it has 5v and still isnt running... time for a new mega
Edited 1 time(s). Last edit at 08/15/2018 02:41PM by Dust.