Civil 3D Labels are annotative by nature, so if you are using a text style in that lable style that is annotative, you are making an annotative style annotative, which in my experience causes problems similar to the one you are describing.

 

If you are working in metric, make sure the contour lable style text height is set to something appropriate (1.5-2.0mm), and if that is correct then check your drawing scale (1:1000 should be fine for viewing lables in model space).  Then check and see if you can see your labels through the viewport or not, if you have corrected the problem in the model space, but can't see your lables in your viewport, then it's a simple page setup units issue.

 

Hope that helps! (even if it is a little late)