Software center has disappeared

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http://askubuntu.com – I'm an absolute newby for Linux / Ubuntu. Yesterday I installed Ubuntu 11.10 on an old computer. It seemed to work fine. However today when I started the computer again the software center had disappeared!! It wasn't in my starter list. When I used the Dash button it wasn't in my application list. In the Dash center you can also look to applications ready to download and there it was, the software center. But how do I download the software center without software center? Double clicking, right clicking, nothing seems to work. How can I get back the software center, and how do I prevent it (HowTos)