It's a little unclear exactly where the confusion lies here - is it in the use of contractions? Past-tense verbs? Multiple definitions for "have" and miss"?
"Had" is the past tense of "have."
"Have to [verb]" in this case is an imperative or obligation like "must [verb]" or "forced to [verb]". See Definition #2 here.
I was forced to miss the meeting due to something out of my control.
To "miss" in this case means "fail to attend," or let something pass by. See Definition #7 here.
I missed my doctor's appointment yesterday.
This is a different use than the idea of "missing" a person because they are gone, but it's similar in that there's something/someone you wish to encounter but do not.
"Won't" is a contraction, or a shortening of a word signified by the use of an apostrophe in place of the missing letters.
It's (it is) short for "will not." This is equivalent to "refusing to."
My car will not start. My car refuses to start.
If "won't" is unclear, then perhaps discussing contractions in general is a good idea.
Other common ones include:
- "can't" for "cannot" (unable to do something)
- "shouldn't" for "should not" (it's a bad idea to do something)
- "wouldn't" for "would not" (hypothetically refusing to do something)
- "don't" for "do not" (telling someone not to do something)