What kind of sentence is ...

where did all the people go

is a

complex sentence, imperrative sentence, interrogative sentence, compound sentence

The answer is interrogative, as interrogative stems from the word interrogate, which is to ask. Judging by the tone of the sentence, I can infer that the missing punctuation at the end is a question mark, which hence leads to an interrogative sentence being the correct answer.

Interrogative. they r asking a question, which is a form of interigating someone.