I think you are talking about the "Kessler effect". And yes, there is concern. The idea is that at some point there will be so much debris, especially in low orbits, that it will keep smashing into itself creating even more debris, faster than the natural orbital decay will bring it down to earth.
It is already more expensive to plan missions to space because of debris. It is already more dangerous for the same reason. Certain orbital paths are worse than others. As to whether the Kessler effect has already happened? I haven't seen too many articles that think it has. But here is something to think about. A 5 cm piece of debris travels very fast in space, and is big enough to wipe out satellites, solar panels....and last I read there were like 20,000 of them flying around right now.