Viewfinder?
If you are very new to SLRs (I'm guessing you might be from buying a D40x, although it's a great camera,) you might be mistaking the LCD screen for the viewfinder, in which case you wouldnt see the picture you want to take on the LCD screen, but instead have to look through the eyepiece (the hole that looks at the mirror that reflects whats coming through the prism from the lens.
If you aren't new and are talking about the eyepiece being screwed up and not working, then I would check your diopter setting, the small little switch right next to the eyepiece you can adjust.
If you could be a little more specific about the "vewfinder" and how it is not working I think I could have a better idea of how to help...
Haha, I had the same thoughts as you Colin, only 2 minutes later. I think a more specific posting of the issue would really clear things up.