Once they are adults it is not uncommon for Horned frogs to go off feed periodically particuarly if there is a change in photoperiod or temperature.

You can try several things to get it back on track,
1) increase photoperiod to about14 hours/day
2) rain the frog with warm water for a couple of days in a row (warning if it is a male this may get him calling....)
3) weigh and monitor the frog. As long as it doesn't lose more than 10% of its body weight, wait until it goes back on feed (becareful when weighing because you can get a false positive for weight loss if the is weighed with a full bladder and is later weighed after it voids its bladder when picked up).

Sometimes even just raising the humidity works.

Ed