I've seen this happen before, where mysteriously the shortcut for the command prompt program vanishes. It should still reside in your System32 folder, though.
Press CTRL+SHIFT+ESC. In the pop-up dialog, select the File menu and "New Task (Run)", then enter "cmd". If it works, then the command prompt file is there. You can enable Windows Explorer to show you system files, then navigate to the folder Shawn mentions above and create a new shortcut. You can create more than one of you wish, in addition to putting one back in the "Accessories" folder where it originally existed.