If it didn't carbonate the first time, the sugar you added might still be there. Add yeast and more sugar and you might have bottle bombs. Or not. (be careful)
Buy a commercial beer with similar character as your homebrew was supposed to be (or a highly-carbonated macro-brew with no character) and mix them 1:1 with your flat homebrew. And as a plus, you get to drink a double each time
I use brands like Milwaukee's Best Ice or Natural Ice to mix with flat beers; they will mix with anything and not water it down too much. This also works to thin-down really heavy "imperial" beers when I got carried away brewing them and they are too thick or sweet or boozy to enjoy. I tend to have more of those than I do flat beers.