Sound is only coming out of one side

First check the cable and plug they can very easily break. Sometimes bending and flexing can cause the sound to appear. Try this all over right to where the cable joins the headphones. The only other problem it can be is a failure of the speaker. UNLESS there's a volume control or a switch in the cable or on the phones themselves. These can break or get dirt on the contacts cutting sound off.
PS Make certain they don't work in other amps first, headphone sockets can breakdown too!
If you find it is the plug, you can cutt it off and fit a new one if your any good with a soldering iron. The cable is much harder as you would need to get into the headphones to replace it. If the speaker has gone you would need to order one from Beyerdynamic.
Dirt can be cleaned out with Servisol Switch Cleaner.