My sennheiser hd 212pro headphones has only the left side speaker working, the other side has no sound. i pulled them apart to see if anything had come lose and nothing has, i dont know wat is wrong with them?

The problem with the replacements are that you keep having to replace them , the snag and brake easily, i have brought four, and wish i just brought a better set.
Any hoo, i have just started to re mod them buy adding a slightly thicker cable and soldering directly to the speaker, which will have a double tie off for when they do snag, i am also cuting out as much of the back as possible to make them backless, it already gives them a crisper sound the bass sounds more tru, and the complete sound is less muddled, they sound $100 aud better

I own a pair of Sennheiser 520II headphones with a similar problem. One speaker element is out due to a broken wire. I think the wire can be re-soldered - if done carefully. The wire is hair thin and you may need a magnifying glass to see it clearly. But it just takes a cheap soldering iron and a lot of patience.
And of course, a small piece of similar wire to reconnect the broken leads (which you can either buy a Radio Shack or cannabilize from any cheap broken speaker and use the wire in it as a replacement).

I'm gonna give it a try, since the speaker element appears to be in very good condition otherwise.

One speaker element may be blown or wiring may have internal break. It may be cheaper to relace the phones rather than having them serviced, unless they are still under warranty