Customize button missing?

[yulaihing]

That darker blue toolbar with the 'customize' button is missing from all my folders.
It used to appear only when i put my mouse over it but now it doesn't seem to exist anymore... I don't remember what I did but is there a way to restore this bar? btw, what is it called for future reference?

thanks!

 

[lemur]

Customize tab is missing in Folder Properties