I recently had the same experience with three beautiful geraniums. While I waited helplessly, one (red) died. I sprayed this week with milk (of all things) and the remaining two are recovering. If that does not work, try an appropriate fungicide. Of course, you should first check the soil to see that the plant is not loose.
Good luck!.

Be patient, your geraniums are just taking a breather.
Plants which produce blooms constantly, can get tired...lots of energy is expended.
Just water normally, don't overfeed, give them lots of sunlight. The blooms will come back as good as before.