might be mites, although a hive that has been around that long and done well usually has developed an ability to deal with mites. mite loads change from year to year, so they may have done them in this year. other possibilities are losing the queen and not getting a mated queen back into the hive, getting into pesticide, or just leaving for a better home. rats might also have run them off or eaten the brood. not much that a rat won't eat. were there many dead bees, or does it look like they just took off?
if you are happy with the location of that hive, just clean it out and leave it as it is. there is every possibility that the site will attract a swarm in the spring. any honey that is capped is probably fine to use and as Allen said, you may melt the wax down for your use. plenty of ways to do it from folks on here. i melt it in a double boiler through cheesecloth, but be very careful not to boil it over. it's quite a mess if you do, not to mention a fire hazard
honey may be taken by crush and strain. also many ways to do that on here. pretty simple and takes no equipment, or just enjoy in comb.
might want to set some rat traps or invest in a few barn cats!