How to enable the sidebar in finder? All out of a sudden, the sidebar in "Finder" is gone. The weird thing is that the "Show Sidebar" entry of the View menu is disabled/grayed out, so the sidebar can't be enabled again. Do you have any ideas how to fix this issue?
Btw, restarting did not change anything.

Edit: Happens on OSX Yosemite/10.10.3
For me, the solution is that I need first (by Apple mis-logic) to hit: 

"View / Show Toolbar" (⌥⌘T); 

…then "View / Show Sidebar" (⌥⌘S) becomes ungreyed.