The Trademark Manual of Examining Procedure (TMEP) is available online at the USPTO website. This resource contains an explanation of excusable nonuse and examples of circumstances under which a claim of excusable nonuse has been accepted. See TMEP §1604.11.
In general, nonuse must be temporary, and the owner must clearly demonstrate how the circumstances prevent use of the mark in commerce and what efforts are being made to resume use. Please note that nonuse due to the decreased demand for a product does not by itself constitute “excusable nonuse.”