Curling leaves are a sign that something is wrong with your geraniums. It could be disease--look at the stem. The most common disease of geraniums is blackleg where the stem starts to rot.
If your plant looks healthy except for the curling leaves it could be a lack of some nutrients or too much of one nutrient like nitrogen--the first number on fertilizer. The most common lack is potassium(potash) or the last number on the fertilizer package.

If you feed geraniums a balanced fertilizer like 15-30-15 they should be fine

Thank you so much.