Sounds like a URI - Upper Respiratory Infection, otherwise known as a "kitty cold".
Definitely needs a vet looked at him/her and medication ASAP. If you have other cats, keep them separated.
I'm not sure what you can do in the meantime before you get to a vet, but I suggest if the cat is an indoor/outdoor cat, then keep them indoors only, keep an eye on fluid intake, and make sure your kitty is still using the litter tray. And give him/her lots of cuddles - you know how yuck you feel when you have a cold
Try googling for Upper Respiratory Infection in cats to find more useful hints!