Another fabulous thing you can do to check that your beloved garage door is in perfect working condition is to test the manual opening system, not just the electric garage door opener. Disconnect your automatic opener- to do this, yank firmly on the release lever or release cord (should be pretty obvious, they're often marked with red or orange plastic). Once you do this, the door should be easily liftable into manual mode. If you're having a hard time lifting it (or it sinks quickly), there is probably something off with the door. Have a professional check the spring tension, as a tightly wound garage door spring is a very dangerous thing.
If, on the other hand, there is a problem with the opener, there are other measures you can take. If your door isn't reversing like it's supposed to when it encounters and obstruction (and you should definitely be checking for this, especially if you have small pets or children) there could be something wrong with the sensors. The automatic reversal function operates via optic infrared sensors that detect when the beam between them is disrupted. If they are not correctly wired or perfectly aligned, or have been damaged, or have some sort of gunk covering them. Clean them up and make sure everything is correctly attached. If that's not possible, or there seems to be permanent damage, you may need to replace them. You'll probably, again, need to contact a professional garage door services person for this, since there parts are notoriously difficult to obtain as a regular consumer.
In general, you probably shouldn't be taking any hardware from your garage door set up without help, because it's hard to fully understand how these parts interact and can react to your meddling, and you might injure yourself or damage your equipment.
Garage door parts have been known to cause concussions and take fingers. Unless you are a semi-pro do-it-yourself-er, you'll need to call in the professionals.