So it is fairly common for cats to get upper respiratory infections- like us catching a cold. Most often there is a viral cause, sometimes they can develop secondary bacterial infections though. Another possibility would be an inhaled irritant or mild allergies. If your cat is eating and drinking and active as normal, then sometimes they can clear the virus on their own. But if they develop trouble breathing, or start to get green or yellow discharge from their nose, have decreased energy or appetite, then they may have a secondary bacterial infection and need to see a vet for antibiotics.
If you feel right now like they just have nasal congestion and aren't having real breathing issues, you could try treating them symptomatically for a day or two. What I would recommend would be to get some saline nasal drops (you can get them over the counter in the human pharmacy- most of the squeeze spray bottles when turned upside down will come out as a drip). You can put a few drops into each nostril a few times a day to help loosen any dried mucus in the nostrils. You can also either put them in a room with a humidifier overnight, or you can put them into a bathroom while you run a steamy shower for 15 minutes once or twice a day to provide some moisture to their respiratory tract and help loosen any congestion. If they still seem congested after that, you can get affrin nasal spray, and put one drop in one nostril in the AM, and one drop in the other nostril in the PM, but do not do this for longer than 3 days in a row.
If despite this,your cat is not improving, or as I said if they develop green or yellow discharge, decreased appetite, trouble breathing, lethargy, then they need to see a vet for oral antibiotics.
Here is an article on cat upper respiratory infection FYI:http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=613