You’ve had that good bottle of champagne longer than you can remember! Dust it off and enjoy it! As long as it has been stored in a cool, dark spot, like your basement, it should be fine! The flavor may have slightly changed from fruity to nutty and the bubbles may be flat, but the bubbly can still be enjoyed because of the high acidity and carbon dioxide which act as preservatives. Store future bottles on their side in a cool dark cabinet and maybe you can use them to toast in 2040!