Tools | Customize from the menu.
In the Build section, drag the "Solution Configuration" item from the list on the right to the build toolbar. You might also want to drag the "Platform Configuration" item as well. The Configuration Manager itself is available from the drop-down list of either of those controls. Also check Tools|Customize|Toolbars to make sure that the Build toolbar is visible.
You might also try the infamous devenv /resetsettings from the command line - that'll completely reset VS to the out of box configuration, so anything that's gone missing should come back.
If none of that pans out, I would guess that your VS installation is damaged somehow and a reinstall is probably in your future.