Many video problems are caused by a slow internet speed or an overloaded browser cache. Symptoms are starting and stopping, garbled sound, and the like. If you are experiencing issues with online videos it's a good idea to clear your browser cache and then reboot your computer to see if that helps. It's also good to try a different web browser like Firefox to see if that will help.
Here's a helpful article on clearing your browser cache:
If you are on a phone or tablet the same thing can happen. Powering the device completely down, and turning back on will give it a fresh reboot. Also, you may need to tap the video enough times to get the device's video player to open up. Usually takes a minute.
A common problem is if you have some other program running on your computer that is interfering with videos running. It could be something that's using all your resources, or a browser plugin that is blocking access. Be sure to check for that as you troubleshoot through problems. Pop-up blockers and Ad-blockers often block things that people really want to see and may need to be turned off.