If you did not do the Qadha' of the previous holy month of Ramadhan, and entered into the next Ramadhan, and you were capable of re-doing those days, you will need to pay a Kaffarah.
The Kaffarah is 750grams of flour, or bread, or rice, or dates, or something similar to these items for each day. You select the amount (750grams) of only one of these items, of your choice.
You will also still need to do the Qadha' of those passed days.
And Allah knows best.