if equipped, check that there are no obstructions in the doorway blocking the sensors path. The sensors need a clear line of sight for the opener to function properly.
confirm that the sensors are aligned and working correctly. Often a sensor will get bumped, knocking it out of alignment. Most door sensors have a green status light indicating when they are in properly alignment.
Verify that there is power present at the receptacle where the openers power cord plugs into. If your using a DMM (digital multi-meter) verify that power is present by checking the hot to neutral and hot to ground. If your not getting the expected voltage you may have a open ground or open neutral.
Check that the circuit breaker hasn't tripped in the main electrical panel. Also if it is tripped investigate the reason why it tripped. The breaker might be tripping because there is a serious electrical fault. Consistently resetting the breaker may damage your equipment and wiring. Just remember that a breaker that trips is likely functioning correctly in that its protecting your equipment and you from harm.
Investigate into the opener's motor torque settings. When a garage door opener is installed, the down force pressure must be adjusted as to prevent an injury to someone if the door was to close on them. By having this setting adjusted properly, there is a reduced chance of a child or pet getting crushed by the closing force of the door. Likewise if the force is set to low, the door will never stay closed and will re-open when reaching the end limit of travel. Also look into the garage door springs. They may need an adjustment or replacement. A broken torsion spring can cause the symptom of a electrical issue in that the door opener motor may sound like its struggling to open. This is because the motor needs the stored energy in the spring to assist with raising and lowering the door.