What to do if my question is downvoted for no reason?

If it literally and absolutely is for "no reason" then you do one, all, or some of the following depending on your personality:

1. Shrug your shoulders and move on
2. Ask in comments why people are downvoting you
3. Ask on Meta
4. Curse and swear
5. Rant in comments

In all cases, I suggest "1", and possibly "2" and "3" depending on the scenario, as the other options do nothing useful or helpful for you or other users.

However:

Assume the question has 20 downvotes and has no comments at all.

I would then also assume that there is a very good reason for the downvotes. One or two downvotes could be nonsense reasons - someone got out of the wrong side of bed, lost their keys, etc.

But 20 downvotes is a sign you need to step back and carefully evaluate your question. There is something wrong, and you need to find it and address it. 20 downvotes also means it probably shouldn’t be too hard to find or know what the issue is, as that's a fair number of users voicing a negative opinion.