いいの? What does it mean?

Question

These are the lyrics

でも　全部わかってしまったらどうすればいいの.

My question regards the ii no at the end. The translation I found was that it means "But when I know everything, what do I do then?" So what does it mean exactly?

Answer 1

I might say

but even if I wound up knowing everything, what should I do then?

My reasons for this different suggestion are two-fold.

1. しまう is a "helping verb" that means "wind up" or "end up"
2. どうすればいい does literally mean "what would be good to do", but generally "what should I do?"

Answer 2

の, with rising intonation, can be used to ask a question. Without rising intonation, it can be used to answer a question. You may wish to research "explanatory particles" which is what this is.

The question you quoted could be answered with:

全部わかってしまったら何もしなくていいの (If you wind up knowing everything, you should do nothing)

The question you quoted is equivalent (though with slightly different nuance / politeness) to:

でも　全部わかってしまったらどうすればいいのか　(question particle か explicitly included)

でも　全部わかってしまったらどうすればいいのですか (polite)

でも　全部わかってしまったらどうすればいいんですか (の contracted to ん, to the best of my knowledge only if followed by だ or です)