A: There are a number of possible reasons.
First you should ensure that the motor unit has power. If your unit is equipped with a plug, make sure the plug is securely connected to the outlet. You can also try plugging a lamp into the outlet to ensure it is working properly. If the motor is hardwired into the home's electrical system make sure the circuit is "live." You may have to check your fuse or breaker box. Some systems also feature a wall switch that must be in the "ON" position for the opener to operate.
Check to make sure that all locking devises have been deactivated. In addition to manual locks on the doors many systems also include a feature that deactivates the motor. This feature is usually activated by a button or switch on the wall mounted controller. Make sure that this lock has not been activated. In most cases activation is evidenced by a blinking light or button.
Make sure that the door is not being restricted from moving by accumulations of ice or snow. If this is the case you may have tripped the overload protector through repeated attempts to open the doors. The protector will usually reset in approximately 15 minutes.
Visually inspect the springs to ensure that they are intact.
If your system is equipped with a remote controller you should check to see if it will operate the system.