How is the grammar working in いや、何かしないとな?

Question

From what I've gathered. The sentence いや、何かしないとな means something like "No, I've got to do something" .

I cant quite understand how the grammar is working here to turn the negative suru verb into a positive sentence.

Answer 1

と normally means that the sentence is a conditional: if you do that, then something happens:

何かをしないと、問題がある。

It's common in Japanese to keep the first part (ending with と) and remove the second part, in which case the intended meaning is "if I don't that, then it's bad", in other words "I have to do that".