This is the simplest solution out of all the ones here.

Solution 1: Checking if other users have the document open Reason 3: If over a shared network, another user could have it open.Reason 2: A second version of Word is already running with the document open.Reason 1: The owner file was not deleted because Word was previously quit improperly - for example, Word crashed.

Sometimes a word document is locked for editing.