Did you check to see if the stems or branches on the sick side got crushed or damaged? Petunias are so delicate that just a little pressure can damage the stems and they don't recover. Maybe you need to cut it back a little more on that side.
If I could see it I might have a better idea. As Tiger said, at least it's perking up on one side already. That's a start!
ajtgirl: I'll say this before i pay for it(I have an account with the Florist i'm bringing it in to her.)I bought another one at the grocery store and it's doing great.Our weather is not good it's 49 this morning for June.What a bummer our garden were just waiting on.