I'm helping a friend whose rMBP 15" is struggling to boot. It chimes and the screen turns on but would get stuck at the grey screen indefinitely. I tried reseting SMC and PRAM. SMC reset seems to respond but the keypress for PRAM does not do anything. I was not able to boot from external drive by pressing Option key either. It seems the second the power button is pressed, no other input is recognized by the computer. If anyone has experienced this issue before or a suggestion, please share. Thank you!