OK, really pointless grammar question, but I thought I’d throw it out.
Where were you? I was worried.
Where were you (I was worried)?
Where were you - I was worried.
I was worried: where were you?
If you feel two clauses are closely related enough to prefer a semicolon as opposed to a question mark, but the first one is a question, how do you phrase it?
Well, just because they’re closely related doesn’t mean they can’t be two separate sentences. I don’t think you would normally use anything other than a question mark at the end of a question.
In this particular example I’d also go with #1. But I can think of other cases where it’s less clear-cut. For example, Nice weather, don’t you think? Most English-speakers would write this as one sentence, when there are actually two components: the statement “Nice weather”, and the question “Don’t you think?”
Spanish-speakers, for example, would write this as one sentence, but they would separate the question from the rest of the sentence, like so: Hace buen tiempo, ¿no crees? The “inicio de interrogacíon” and “fin de interrogación” (beginning and ending question-marks) clearly denote the question. Neat system they’ve got going there.