Software updater doesn't display Ubuntu 14.04

Having the same problem, I found I can click on the icon in the tray once and not see anything happen, but click on it one more time and the window appears as it normally would, just like clicking the icon once to minimize the window and clicking again to re-display that window; annoying but slightly faster than restarting Update Manager from the menu.

This has been reported as a bug on Launchpad:

https://bugs.launchpad.net/ubuntu/+source/update-manager/+bug/1325814?comments=all

It was a workspace-related issue for me. I have 3 monitors arranged such that they don't add up to a rectangular X-screen. When I press Super+S to get the workspace switcher, I can see the "invisible dead space" in a corner. The software updater had the misfortune of being opened in one of these dead corners!

+--------+--------+
|        |        |    *SU*
|        |        |    
|  Scr1  |  Scr2  +------------+ 
|        |        |            | 
|        |        |   Scr3     |
|        |        |            |
+--------+--------+------------+

 *SU*  = Software updater window

From the switcher, I was able to drag it to a visible location.