If your door will go up but not down, check the electric eyes at the bottom of the door. They may be covered over with a leaf or spider web, or someone may have knocked them out of alignment. Make sure the indicator light is lit up on each one, the path is clear, and they are "seeing each other" and your problem is fixed!
Your car is either equipped with Homelink or Car2U. The easiest way to tell what you have is that Homelink typically has 3 buttons and one light (see picture above). The Car2U has a light above each individual button. FYI, most Car2U vehicles need a "compatibility bridge" to work with Liftmaster/Chamberlain motors.
The link on how to program Homelink is below. Cars equipped with Car2U should refer to the owner's manual of their car.
Instructions for most keypads are located inside the flip up cover. If your keypad is not working all-of-a-sudden, check the battery. If that doesn't solve the issue, try re-programming following the instructions inside the cover.
Remotes are typically easy to program. Locate the "learn" button inside the light cover of your motor inside the garage. Press the learn button ONCE (do not hold it or you will clear everything out). After you have pressed the learn button slowly press the button on the remote until the door operates.