The issue is with your browser and/or computer. It's difficult to diagnose with just your short description. If it worked before and now it doesn't:
Did you install ANY new application or upgrade one?
Did you modify your antivirus or firewall settings?
Can you go to any other site and play their files?
Do you have another computer on your (home) network that is able to play it?
Are you at work? Maybe your work has added restrictions.
Try deleting your browser cache file-
(In Internet Explorer it's: Tools/Internet Options/Browsing History/Delete)
Try updating your browser (Help, About, Check for Updates)
Try installing and using a different browser, like Firefox