If I were you I would also break off some pieces of healthy
plant at the joint and stick them in a different pot of potting soil... These pieces will root and give you more
plants...Just in case you cannot save the
plant that you love...You can at least have pieces of it...Geranium is EASY to root.
You might want to re-pot it so you can pull out the soil and see/and or smell if there is rot in the soil. If there is you need to add as much new soil as you can. It should come back Geranium is a very resilient
plant.
There is a topic on here rooting Geranium <I think>> that may also be able to help you.