This happened to me a few years ago with our Coleman family tent. It got rained on heavily, but we "thought" it was dry before we packed it up. We took it out a year later and it smelled to high heaven. Here's what I did to fix it...and it wasn't fun.
First buy this product from Nikwax to wash your tent.
http://www.nikwax.com/en-us/products/productdetail.php?productid=268&activityid=1If you can fit your tent in a front load washer...great, but mine was too big. So, I set it up in our backyard and made a solution of the Nikwax soap in a bucket. Armed with a soft bristled toothbrush and a water hose I attacked every blackened spot on the tent until 98% of it was gone. I then rinsed the tent off completely and let it BAKE in the hot sun until evening.
Once it was completely dry I applied a waterproofing product.
http://www.nikwax.com/en-us/products/productdetail.php?productid=252&activityid=1 I have used the tent for the past two summers and it has been perfectly fine since then. It was a lot of work, but worth it to me because I love that tent.
Hope this helps!
Jeff <img src="/forums/images/graemlins/grin.gif" alt="" />