First, make sure your device is up to date. Chromebooks and Windows devices run updates when they are shut down. Be sure to use the Shut Down feature, Closing the lid does not shut down your device.
Also, if you are on Windows, your browser may need to be updated. On Chrome you can type chrome://settings/help in the url bar and press enter, then click on the link "To see if your device is up to date, go to Chrome OS Settings" - to finish updating your browser, if needed, you will need to close the browser and open it again.
If your video still isn't playing, you can download the video to your device, then you should be able to play if from the downloaded location. If this is the only way you can get the video to play, or if it doesn't play even after downloading, please use this, but also create a new ticket so we can help you troubleshoot further.