Why can’t my lens focus whether on auto focus or manual?

There are four potential causes I can think of:

1. Detached Focus Key: The focus key is a metal fork that is attached to the main focus ring. The focusing lens has a plastic shaft attached to it which rests in between the two metal forks of the focus key. The effect of this is that whenever the focus key rotates, the focusing lens rotates with it. The focus key itself is unlikely to break, but that plastic shaft connected to the focusing lens could have broken off. It's also possible the screws holding the focus key in place came loose. In either of these cases, you'd likely be able to hear something rattling inside of your lens, unless they somehow lodged themselves in place somewhere inside of the lens.

2. Improper Reassembly: This cause also deals with the focus key and can only be possible if the lens was disassembled prior to the issue occurring. If the lens has never been taken apart, you can ignore this one. Otherwise, it's possible that when the autofocus assembly was being reinstalled into the lens, that the focus key was not properly aligned to the plastic shaft on the focusing lens. When reinstalling the autofocus assembly, the plastic shaft on the focusing lens has to slide in-between the two metal forks on the focus key.
3. Loose Retention Ring: To explain this cause, you must first understand how the focus assembly works. The focus assembly is comprised of three rings.
   The top ring is the autofocus ring. This ring is moved by the autofocus motor.
   The bottom ring is the manual focus ring. When you turn the rubber focus ring on the lens, this is the ring that turns.
   The middle ring is the main focus ring. This is what the focus key is connected to, which is what causes the focusing lens to move.
   Both the autofocus and manual focus rings move the main focus ring through friction. This means that if there is not enough tension, the friction on the rings is reduced and may cause them not to be able to move each other. This can be caused by the tension ring rotating over time and eventually coming loose. Directly below the retention ring is a spring that is also used to keep tension on the focus rings. If this spring wears down, it can also cause tension issues.

4. Optical Issues: There's a possibility the focusing mechanics are actually working exactly as they should. One of the lens elements could have somehow moved out of its correct position, causing the focus to be completely off no matter how much you try to focus it. For this to possibly be the issue, you will notice that when you try to focus the lens, it does make a very small difference or, by looking through the front lens element, that the focus lens element is in fact moving when you try to focus the lens. If neither of those are the case, then this is not the issue with the lens.