Sounds like she may have an upper respiratory infection (URI), which is usually virus based (herpes or calicivirus). Is she eating okay? Cats with stuffed up noses tend to go off their food because they can't smell. It's very important that she eat, so feed her the stinkiest food you can find (fish based is often the strongest smelling). You could also go in the bathroom with her while you run a hot shower for 10 mins, 3 or 4 times a day. Hopefully the steam will help loosen the gunk in her sinuses. If you can get to a health food store to get some L-lysine powder, add about 500mg twice a day to some wet food. Here is more info for you on treating URI's:
http://www.holisticat.com/uri.html
I hope she gets better soon!