Satellites eventually wear out. Usually the first thing to go is the thruster fuel. Because satellites drift over time, small rockets are used to keep them in place. When those rockets run out of fuel, the satellite is finished with its life even if all the other components are working fine.
When a satellite has reached its end of life, its owners have two choices. They can tell the satellite to sink to its death back on Earth, but that’s actually more expensive. It takes a lot of fuel to put a satellite into position where gravity will do its job, and that could shorten the life of the satellite by up to a year.
So, most satellites are sent not to Earth but to a parking orbit a few hundred miles further away from Earth. There they sit, and will probably sit forever. Is that a problem? Possibly. As these out-of-control satellites crash into each other, they crumble into smaller and smaller pieces that fly through space. If any of these pieces hits a working satellite it could damage it completely.
There’s a theory that eventually all this space junk will get so dense that it will be impossible to leave the planet. This is called the Kessler Syndrome, but not to worry — it’s not likely to happen soon if it happens at all. That’s the good news. If it looks like our own satellites are keeping us trapped on Earth, we have plenty of time to figure out how to blast ourselves out.