Welcome to Android Central! Which device do you have? I assume you're talking about the YouTube app, and not the website, right? And you're talking about the progress bar, along with the pause/play button? What happens when you tap the screen while the video is playing?
Make sure your YouTube app is fully updated to the latest version. Also go to Settings>Apps>All (or the App Manager, if you have a Samsung device), select YouTube, and Clear Cache/Data, then Force Stop. Now try again.