Always better to use FileNameJoin
than to type in the full path name as a string. Further, you should give the name of each directory in the path individually. So you should use
NotebookEvaluate[
FileNameJoin[{Directory[], "Documents", "Mathematica", "SomeFile.nb"}]]
An alternative is not to specify the notebook file, but select it from a system open dialog. This can be done with
Module[{path},
path = SystemDialogInput["FileOpen", "*.nb", WindowTitle -> "Select a notebook"];
If[path === $Canceled, $Canceled, NotebookEvaluate[path]]]