Suddenly my DVD player is saying "no disc" after trying to load DVD. Why isn't it sensing the DVD has been loaded?

When a DVD goes into DVD player it has to spin it and read it. If your DVD player is not reading the disk. The disk could be dirty. The DVD player might need to be cleaned. You could be trying to play a PAL disk on a NTSC player or Visa Versa. or the region coding might be wrong. Try playing a clean disk that you have played in it before. That might tell you if you need to clean the DVD player.