Hello. I appreciate your help. For two days I can't work with You Tube. When I click on the most of the videos, that I used to click before, appears just black rectangle without any buttons, without anything. How can I fix it?

Hello,

See in each Browser which version of Flash it thinks it has...

http://kb2.adobe.com/cps/155/tn_15507.html

Then Test Adobe Shockwave & Flash Players...

http://www.adobe.com/shockwave/welcome/

See if this might be it...

Out-of-date versions of Adobe Flash Player do not include the latest security updates and will be disabled to help keep your Mac secure. If Safari 5.1.7 or Leopard Security Update 2012-003 detects an out-of-date version of Flash Player on your system, you will see a dialog informing you that Flash Player has been disabled. The dialog provides the option to go directly to Adobe's website, where you can download and install an updated version of Flash Player.

Additional Information

If you need to re-enable an out-of-date version of Flash Player, you can do the following:

1. Navigate to the /Library/Internet Plug-Ins (Disabled) folder.
2. Drag "Flash Player.plugin" into /Library/Internet Plug-Ins.
3. If the browser is running, quit and restart it.

http://support.apple.com/kb/HT5271