It's easy to drag a start menu item inadvertently and when you release the mouse button, it goes no telling where.
The path to it is %systemroot%\system32\rstrui.exe so look there and see if it's not there. If so (which I expect to be the case), you can create another shortcut to it.
You could also click the Start button and type RSTRUI in the search box and get to it that way.
Good luck.

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