Outdated Applications and Platforms
When watching videos or other streaming content in your browser, keep your software up to date to make sure it can play what you want to watch. In some cases you may experience problems if your browser hasn't been updated, but you should also make sure that Adobe Flash is up to date (see link in Resources). If it's not, you may only see a black screen where your video should be, and your sound may not work correctly either.
Unfamiliar File Types
If you're trying to watch videos you have downloaded or imported onto your computer, hover over their icons or right click and select "Properties" to see their file types. If some videos work and others don't, the latter videos may be in a format that none of your current video players support. This can happen often if you're using only your computer's default media player. While you can try to find the files in different formats, you can also download media players that support a wide range of formats, like VLC, Wondershare Player or UMPlayer (see Resources).
Security Software Interference
Security software like firewalls can interfere with attempts to stream or play downloaded media by blocking any files or websites the firewall views as a threat. Not all software will notify you when something is being blocked, so disable your firewall or security software, then attempt to to watch your videos. If the problem goes away, you've narrowed down the problem to your security software, and can then set exceptions for specific websites or programs.