Attempting to explain downvotes can be a tricky thing (unless you happen to be one of the downvoters).
Just to be clear, I didn't downvote your question, so everything that follows is only speculation.
I found nothing wrong with the format of your question, and I think you provided sufficient context.
Sometimes people will downvote questions that seem too basic. Maybe someone thought your question was too basic, or maybe someone thought you were asking two basics questions that didn't seem connected. Specifically, I did wonder about your last question, which muddied the waters some:
Is "No" being used as an interjection here?
You started by asking about punctuation (should I use of a comma or a semi-colon?), but then you concluded by asking about parts of speech. This made the question more complicated, which might have prompted a downvote.
How does your last question complicate matters? Well, either "No" is being used as an interjection, or it isn't. If it is, how does that answer your punctuation question? If it isn't, what's the answer?
Had you explicitly tied the "is it an interjection" question with the "how should I punctuate this" question, you might have fended off some downvotes (assuming my theory has any validity, but, again, I'm only conjecturing). So, you might have added:
If "No" is being used as an interjection, then I assume you use a comma. But if it's not an interjection, then it stands on its own as a sentence – maybe a semi-colon would be correct?
As a side note, if "No" was in fact an interjection, I would punctuate it differently altogether:
No! The new software can still open the old files.
However, I don't think it's an interjection (but that's not a matter for meta).
In short, this jumbling of these seemingly basic matters may have made it harder to answer the question, which may have prompted a downvote. I didn't downvote it, but I opted to supply an answer instead. I hope you find that answer helpful.