If you are separated from someone in a crowded place, it would be more common to ask, "Where did you go?" or "Where did you disappear to?" If you haven't seen a friend in a long time and you want to know why they have been away, it would be more common to ask, "Why haven't I seen you?" or "Why have you been away?" "Why do you suddenly disappear" sounds odd because if you can see them again, your sentence should be in the past. "Why DID you suddenly disappear". I hope this helps.

It depends on the context. The sentence by itself sounds incomplete. If you want to use the word "do" and not "did", then you should write it like so: Why do you suddenly disappear when... Why do you suddenly disappear whenever... Why do you suddenly disappear if... If you are not using the words if or when or whenever to complete the above sentences, and you just want to express a completed thought then your original sentence should read: Why did you suddenly disappear? Hope this helps!

Why did you suddenly disappear or Why do you suddenly disappeared

But the first one is more acceptable I'd rather use the first one