I was trying to get a smooth, frothy peanut butter frosting. I used a cup of creamy peanut butter, half a cup of butter, and 2 cups of powdered sugar. I liked the consistency of the mixture. But I accidentally left it on the stove and the heat of the oven caused the frosting to melt. The sugar is crystallized, and the color is much darker now. Also, its a lot thicker than I want it to be. What do I do now? Is it ruined, or is there a way for me to rescue it?
There is no way to rescue your frosting. It is not just a mixture of ingredients, it used to have a very complicated internal structure, which is now gone. You will have to start a new batch.
I assume that the taste of the frosting hasn't changed much. It cannot be used for its original purpose, but you should be able to repurpose it in many ways, for example as a bread spread, or as a filling somewhere.