when I put in my headphones into my macbook pro only the right side works. this just happend. Whats wrong with it and how can I fix it?

I know this has been resolved, but just for others seeking solutions, I headphone cable actually plugs in on both ends.. meaning, they have a plug into the actual headphones themselves.. so, at some point in tucking them away, I unplugged the cable all the way and when I plugged it back in, thought they were broke or required an adapter because only one side was working. As I almost ordered an adapter for them, I realized one end had three black lines on the brass prong while the other had two.. when I plug the three black line end into the headphone end, all back to normal. Simplest thing to miss and almost didn't catch it.

Is the jack fully inserted into the socket? Is there visible damage to the headphones or cable? Do the headphones work normally with another audio device? Do other headphones work normally with your MBP?

Open System Preferences -> Sound -> Output. Is the balance control pushed over to the right?

There are three sections on the plug, one of them is not making a physical connection to the Mac, likely inside the Mac's port.

Use a light bit of rubbing alcohol on a thinned down Q-tip and clean both.

If that doesn't work it's possible the plug is bent a little or the contacts inside are not functioning anymore.

Your getting common ground and one speaker, so two channels are working, sometimes rotating the jack in the socket works.

It could be that one of your headhphones speakers have failed or the volume isn't turned up enough.

HA! excellent. it was the balance, but that's weird it just happened out of nowhere

You know, not so weird. In the last year or two, I've discovered that my balance has occasionally shifted — certainly not something that I intended.

I suspect we're doing something to alter it, but god knows what.