When you start an action that doesn't finish quickly, PowerPoint displays a message box that has a Cancel button. A progress bar may also appear in the status pane at the bottom of the PowerPoint window. Examples of long-running actions include saving a file, inserting a large video or audio file, compressing all the photos in a large presentation, or replacing fonts in a presentation.
If you have changed your mind and do not want to do the long-running action, you may press ESC or click Cancel, in which case the operation in progress will be undone. If the action you are waiting on seems to running indefinitely, it's probably best to cancel the operation and try it again.