I had an issue with an app being stuck in Paused mode in Launchpad, and was unable to cancel/resume the update from the App Store because it wasn't listed in the updates list.

The solution, as stated here, was to open Launchpad, hold down the mouse on the app until it started jiggling, then clicking the delete button (x). The app reappeared normally and was even updated correctly.

The reason was probably the app got updated along with a system update, and Launchpad was unaware the update was completed.