Do you see any errors or warnings in the application log?
If not, then most probably the segmentation’s geometry does not overlap with the image. This happens for example, when you create the segmentation with a different image than the one that you are trying to segment. You can fix it as described here or re-create the segmentation (for example, save the image as nrrd, restart the application, load that saved nrrd, then go to Segment Editor).
Can you share a sample data so that we can investigate the DICOM import?
If you just need 2D area measurement then placing closed curve markups and enabling area measurement might be easier than creating segmentation.