The Missing files status (displayed in red) is new in version 3.2.0 onwards, this indicates that all, or part the payload is not where qbittorrent expects to find it. One task in this status may be caused by a file having been deleted or damaged by external means, such as a virus scanner, multimedia 'tagger' etc. If ALL loaded tasks are showing this status is indicative of a drive not being mounted, an external device unplugged or a network location being off-line.
A single task in this status can be rectified by rechecking, then resuming the task. If all tasks are showing "Missing files", shut down qbittorrent completely with File -> Exit, remount or re-attach the missing drive, check that the drive has the same mount point or drive letter as previously then restart qbittorrent and resume the paused tasks (Ctrl+A -> Resume or Ctrl+Shift+S)