A red bar at the top of a clip means that it has been rejected. A question mark with a black background means that iMovie is not reading the link to the original clip or that the original source clip is missing.
Try importing the clip in a different way and see if that cures it. Confirm that the clip is in an iMovie supported format.
Also, you can try deleting/resetting preferences. to delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. (To reopen your old library close iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon. iMovie will open in that library.) Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
--Rich