There are a number of reasons why videos might not play, give MX player a try, (link below) and see if this app resolves the problem. Another option, clear the Gallery cache. Go to: Settings > More > Apps (or App Manager), swipe left to view ALL apps. Scroll down to and tap Gallery. Scroll down to and tap 'Clear data', then tap OK. Launch Gallery, (it may take a few seconds to load thumbnail images).