Yes you were a little late with the water and the plant was stressed. When stressed they drop they're flowers, buds and some leaves. Fear not though, they are forgiving. With proper care it will recover just fine but may look leggy. If you don't like the leggy look you can trim off the leggy branches. If you trim the cuttings to about four inches, remove the flowers and buds and then put them in water they will grow roots in about 2 weeks. Plant the rooted cuttings and you'll have more of these wonderful plants!

Don't worry. A few days ago I put my impatien to the front yard (because it was blooming and I liked to display it) where it got boiling hot afternoon sun for a few hours. And I forgot to water it that day (bad idea) and the next day it was going through another round of boiling sun when I realized what I'd done. I quickly took it home, let it cool down, and watered it. It lost a bunch of flowers and the leaves started yellowing. I put it in the backyard (mostly shade). It lost a lot of the leaves. Oh, well, I'll wait. Now I notice new shoots coming up from the base. If your impatiens is leggy, like mentioned above, trim it, but you can also wait for new shoots. In a while, the impatien should be back to normal.