My MX-850 sometimes does the same thing.
Remove the mini jack from the MX-850 and plug it back in.
If that doesn't reset it, reboot your computer and try again.
Both methods have worked for me when my MX-850 decided to be stubborn.
The quick test to see if your MX-850 is communicating with your computer is to hit the Help button at the top of the screen, and then the Firmware Version link in the drop down box.
It will instantly communicate with your MX-850 and give you a firmware version if the connection is good.
If it didn't link to the MX-850, it will give you an error message.
If that happens, follow the two steps mentioned above.