When something lands with a heavy, dull sound, you might describe the sound as kerplunk. A rock thrown into a lake goes kerplunk as it enters the water.
The word kerplunk is an example of onomatopoeia, meaning it is a word that describes a sound by mimicking that sound. It's especially used to represent the sound of a heavy object falling into liquid, but it can be used for other thudding sounds, too. Figuratively, if someone says that something went kerplunk, they mean that it didn't go over very well. If your presentation went kerplunk, it probably wasn't very engaging or successful.