Below is a shot of the main menu and the bottom item is the software center. Make sure check box is selected. Selecting the Line where the software center is listed will open properties also displayed. By selecting the icon image on the left you can navigate to the folder of the icon set in use and open the path to the launcher icon. You may have log out and back in.
If the main menu entry was deleted select + New Item and add the information in the code tags.
Code:
Type: Application
Name: Ubuntu Software Center
Command: /usr/bin/software-center %u
Comment:Lets you choose from thousands of applications available for Ubuntu
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