What is possible depends on your perspective and how much you know. First consider what appears possible before the die has been rolled. If you know the exact physical configuration of the die (and the table, air, etc) as it begins to roll, then in theory you could calculate what number it must come up as. So from that perspective, only one number is possible, because you can cognitively rule out the other numbers.
But if you don't know the exact physical configuration of the die, then you have no way of predicting what it will come up as. From that perspective, any of the numbers are possible, because from your limited knowledge, you can't rule any of them out.
But what about after the die has finished rolling and you know it's a 6? At that later time, you can rule out the other numbers. However, when you talk about what "was possible," we can interpret this as what couldn't be ruled out based on a perspective P prior to the die being rolled. You can rule out the 1, 2, 3, 4, or 5 now, but you couldn't have ruled them out prior to the roll, so it is in that sense that we say those rolls "were possible."
Now, there are different perspectives P a person might have, prior to the die being rolled, and therefore there are different answers for what was possible. When you say what "was possible," you are implicitly invoking a certain P. I mentioned the perspective P1 in which a person knows all the physical parameters of the die and can calculate the physics of the roll, and the perspective P2 in which a person does not have that knowledge. Usually when we talk about rolling a die, it's understood P2 is the implicit perspective invoked. From P2 any roll is possible. But you could, if you specified, say that it was impossible for the die to come up anything but 6, based on a perspective of P1. These are not in contradiction, because they each only refer to a certain perspective. "'5 is impossible' follows from P1" does not contradict "'5 is possible' follows from P2."