I applied the latest OSX Update (2015-002 1.0), and I thought I had bricked my MacBook Pro when it refused to reboot. Symptoms were – Power On, Dvd Spin, No Chime, Power Off. I opened it up and checked to see if there were any obvious internal issues, and I reseated all the cables.
No Luck, until I found this iFixit post:
Try Resetting the PRAM Resetting PRAM and NVRAM
1. Shut down the computer.
2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
3. Turn on the computer.
4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
5. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
6. Release the keys.
Your computer's PRAM and the NVRAM are reset to the default values. The clock settings may be reset to a default date on some models.