What do I need to do to make the dogwood pink again?

What color is it now? I believe they are like Hydrangeas and sometimes need more or less acid in the soil, etc.

I had no idea that dogwoods would do that too, but it all has to do with the pH of the soil. With hydrangeas you can add aluminum sulfate to the soil to make it more acidic and turn the flowers blue. Dogwoods are also acid loving trees. You can get a pH test kit from a local garden center and test the soil around the tree. If the soil is too basic-- add aluminum sulfate. If it is too acidic (remembering that dogwoods like relatively acidic soil) add lime. I highly doubt it will be too acidic so you're best bet will probably be aluminum sulfate if you want to revert the color change. A pH test would be the best way to determine this however.