We need more details.. what did you try to open? what was the original default that you used to open it with? Do you mean that you can't do anything on your computer other than load a browser and surf the net?
you can right click again, and at the bottom of the chose open with menu it should say "choose program." that opens a window where you can choose from a list of programs and there is a box at the bottom to check to make your new choice the default for that file type