Coolscan IV ed No Image when scanning slides

I have a Nikon Coolscan IV ED which has served me very well for 6 years. Last week it would no longer scan color slides. (scan works but there is no image) It will scan color negatives and will scan B&W negatives although the B&W are not as clear as they used to be (grey scale does not have as much contrast). Are there trouble shooting steps I can follow to determine the problem. I'm wondering if one of the LED's are out.

Thanks,

Dave