If a woman gets her tubes tied but they are not clipped & burned is it possible that they can come undone after some years?

Maybe. By undone do you mean go in and have surgery to undo the first surgery, and have children again? Then sometimes it can be "undone" the less destructive the original surgery to the tube the easier it is to reverse it. If you mean can they grow back by themselves very unlikely. The connection is made but often a poor connection between the two sides of the tube causing a "tubal pregnancy".
Yes. You have to have it surgically reversed, and that is not a benign procedure, as it can cause problems with tubal pregnancies.