Video has always been LR's stepchild, mostly neglected by Adobe since it was introduced in LR 3. Often video works fine, but sometimes the playback stutters, audio goes away, or the current version of LR starts balking at importing certain formats.
If/when the various suggested troubleshooting steps don't help you, you might consider what I do: Catalog all my videos in LR for organizing with my photos, but play them back in whatever default player you've got configured on your computer. To play back a video in the catalog, right-click it and do Show In Finder, then double-click it.
If that becomes tedious, use the Open command of the Any File plugin to quickly open any cataloged file in the default app for that file type. I've got Open assigned to Ctrl + O in my LR. The Open command is free (the rest of the plugin costs money after the free trial expires).