UA's FRA-DEN is only a seasonal service.
It will not be coming back this year as a result of lower air travel across the Atlantic. It might come back next year is what I've heard. It all depends on how much the demand for air travel across the Atlantic picks up.
United code-shares on Lufthansa's daily year-round service, so its not too bad that UA's DEN-FRA service will not be coming back this year.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran