When you have a situation for which there is only one choice, that you have to do. You don't really want to do it, but there's no choice.
As opposed to a dilemma when you have two choices for a situation.
"Last night I had to go see the 12:01 showing of Green Hornet. The only time I could see it. I didn't want to, but I had to. Eh, I'm in a real lemma." Ben Mankiewicz, the What the Flick?! movie review show