This worked for me, from the Motorola site referenced before, originally developed by Cubenoob, and further refined by Doc and others:
1.) Go Settings
2.) Sound & Display
3.) Scroll to the bottom
4.) Locate the SMART SENSOR SETTINGS:
A).Double-Tap to Silence = No Checked
B).Smart Profile: Face Down = Checked
5.) Remove and replace the battery
6.) Turn the phone back on and DO NOT PRESS ANYTHING
7.) Hold the phone face up in your palm.
8.) Call yourself from another phone as soon as it fully boots up
9.) As the phone is vibrating, DO NOT ANSWER. Press a volume rocker to stop the vibration but do not end the call.
10.) Still holding the phone, flip the phone face down.
11.) Then, flip the phone back over so that it's facing up. You should feel a quick vibrating jolt. That means the ringer is back on.
12.) End the call from the phone you are calling from.
13.) Don't touch anything, leave phone face up and call yourself again.
14.) If it worked turn off the Smart Profile: Face down setting
If it does not work on first attempt, try steps a few times before resetting the phone.