Okay this is a two step process.

1st: In windows explorer or OSX Finder you need to locate the deleted shelf. The file name should be "shelf_Curves.mel.deleted"
Rename this file by removing the ".deleted" so it should now be "shelf_Curves.mel" Take note of where this file was, because you have to navigate to it again.

2nd: Back in Maya, to the left of the selves, you should see a folder icon with a down arrow below it. Click the down arrow and from the pop up menu, select "load shelf... " Navigate to the shelf file and select it then click open. Your shelf should now be restored.

Note: it will be at the end of the shelf row, if you want to move it, middle-mouse-button drag it's tab to new location on the shelf.