You’re using WordPress and know the Appearance->Widgets menu? On the left side there you have a list of all available widgets. The widgets which aren’t already added to a sidebar – or another element albe to contain widgets – should have an Add-button, to add them anywhere. The widgets that are already added can be removed from their corresponding element and then the Add-button reappears.
Not for me. 
. Many widgets (like Pages, Meta, Links, Tag Cloud, …) just did not have this Add-button – it was missing although I did not add them anywhere. Because of this I couldn’t remove them from anywhere to regain the Add-button. But I wanted to add them! Grrrrrrr! Vicious circle!
After freaking out shortly I gave up. A whole day of recreation and I got the solution:
While playing around with themes some time ago, I tried out the Thematic theme for only a minute. It turned out that it had automatically added widgets to some of its elements during ‘installation’. They were still added there! I switched to the Thematic theme, could remove all widgets from its elements and switched back to my theme. All missing Add-buttons reappeared! Hope this helps.