A "retrospective" power calculation is one that is carried out after the study has been run.
If you haven't completed the study yet, you don't need a retrospective power calculation: you need a proper power calculation.
If you have completed the study, but answered "Don't know" to Question 1, then you don't need a power calculation: you need a significance test.
Click here for help with proper power calculations.
Click here for another retrospective power calculation.
Click here for help with significance tests.