It really depends. I think the SELRES promotion boards are very forgiving for these circumstances and understand people take breaks before deciding to serve again. Vice Admiral Burke (CNP) is interested in Sailors coming back into the Navy and Navy Reserve, both enlisted and officer. These individuals might have acquired skills from the civilian sector who can bring these abilities to their teams.
I concur with a simple letter to the board. No need for a long "Forrest Gump" style story, but at least make sure the board understands the gap. The OP might get FOS this board, but assuming he can get a solid FITREP next cycle should hopefully have a better chance the second time.