I have a love/hate relationship with RealPlayer. It will stop working and sometimes require a reboot or a re-install. As much of a pain-in-the-keester as it can be, I haven't found anything better for downloading videos from tube sites.
Try reinstalling it but I'd actually suggest you uninstall it and never use it again. Instead maybe use VLC Media Player, it's far less annoying than RealPlayer. http://www.videolan.org/vlc/index.html
I agree with spirit. I gave up on RealPlayer and switched to VLC about six years ago.
For video downloading, I use YTD Video Downloader, which is free and works on tons of sites.