Didn't win War (Kebir Blue)

I was fighting a C3 and only had 1 country. I should have won the war when war index dropped 25.00 but I never did. Now I log in the next day to see that country mocking me with its index back up to 28.50

If this was the first war you have ever declared, then you should have won when/if the index dropped below 25 points.

However, in any war, you should expect the index to repair itself a little every month that you don't continue to attack. Countries rebuild fairly quickly.

Well it was actually a second war. So does that mean I have to get it to 0?

Yes.