I had this problem as well with an 820i and an iMac running 10.11.5 (El Capitan). I got it working again by doing an SMC Reset & PRAM Reset.
To do that, I followed these instructions:
SMC RESET
Shut down the computer. Unplug the computer's power cord from both the wall and computer. Wait 15 seconds. Attach the computers power cable. Wait another 5 seconds and press the power button to turn on the computer.
PRAM RESET
Shut down the computer. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4. Turn on the computer. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears. Hold the keys down until the computer restarts and you hear the startup sound for the second time. Release the keys.