due to something fishy ,
one of filenames has entered as
Abhishek�_xyz.docx

and it is not opening now.
It says no file found.

When I try to do rename it using 'mv'command,
then it do not allow me by stating
cp: cannot stat `Abhishek �_xyz.docx': No such file or directory

How can I change name of this file then?

Try by adding a backslash:

mv Abhishek\�_xyz.docx newname.docx

Or with double quotes:

mv "Abhishek�_xyz.docx" newname.docx

Or by using a wildcard:

mv Abhish* newname.docx

By the way which kind of shell are you using? Bash?