Manufacturers say the average life expectancy of an opener is 15-20 years. Since new openers are inexpensive, if the unit is over ten years old then it might not be worth repairing and it may just be time for an upgrade. If it's newer, then it's more likely a repair will be worth it.
If it's higher than four feet off the ground, or lower than three feet, you'll know it needs to be realigned. Use a ladder as a brace to keep the door safely open while you're working on it. This also takes the pressure off the springs so you can inspect them for rust and warping.
The causes of your garage door sticking could be that its rollers and hinges simply become stuck or that the track is dirty or clogged. If your garage door's rollers and hinges are stuck, try applying some lubrication such as WD-40. This should get your garage door back to opening and closing smoothly.