i'd agree with everything sphinx posted except:
-i would change as much water as you can IMMEDIATELY. all but perhaps 6 inches of water, to save some good bacteria in the old water. i don't think fish ever "get used to" nasty water, and i doubt changing to fresh, clean water would make them sick. the only two things to keep in mind - check the temperature and make sure that the water you're adding is the same temperature. if it isn't,
that could certainly shock them. also, use dechlorinator. you might be able to find it in large quantities in the pond section of any pet store.
once you're there, here's what you'll need:
-test kit. one that tests for pH, nitrAte, nitrIte, ammonia, chlorine - at least. more tests are fine, but these are essentials.
-filter or at least large pond pump. make sure it has no sharp edges or places where fins could get sucked in. this will get oxygen into the pond. try to find a pump that sucks water from the bottom of the pond and sprays it across or onto the surface. a good rule is if you can hear the pond, you've got enough oxygen in it.
-a large pond net. once you have clear water and you can get a good look at these guys, you may find they have quite a few health problems - bacterial infections, parasites, fungus, etc. if this is the case, it will be important to net them and get them into a rubbermaid tub or other smaller body of water to inspect them thoroughly and determine the exact cause of the problem. many koi medications are injected, depending on the size of the koi, and having them in a small container aids the medicating process.
-as mentioned, you'll also need koi-specific food. they may not have been interested in the food you supplied because they don't recognize it as food. koi and goldfish are notorious for ignoring new foods for quite a while until they realize they're good to eat! koi pellets or sticks will work. you can find koi food at any local fish/pet store.
once you change the water, you'll need to test it for all the parameters.
your tests should read as close as possible to this:
pH - between 6.5 and 8
nitrite - 0
nitrate - 0
ammonia - 0
chlorine - 0
if you don't get these readings right away, don't worry. perform large water changes once each day until you have these readings. each time, make sure to dechlorinate and make sure not to shock the fish with large temperature changes.
these links will help you:
http://www.vcnet.com/koi_net/
-general koi and pond information. a GREAT resource.
http://www.koivet.com
-good for sick or injured fish, but their "cool stuff" section is a godsend.
http://www.koicrisis.com/
also good for sick fish or a dirty, poorly functioning pond. probably worth looking at in your situation.
http://www.koisupplies.net/
a great place to find food, filters, dechlor, test kits, just about everything relating to koi.
http://www.hartzpro.com/koi_feeding_assessment_koi_food_labels.html
an EXCELLENT site for determining what a good food would be and how much/how often to feed.
and if you have any more questions, we're always here! good luck and keep us updated.