Both are okay, however there may be a subtle difference in the two.
The first: "What will happen...?" may imply that losing the ball is a rare occurrence, and you aren't sure whether the person you are asking will know the answer or not.
The second: "What happens...?" may imply that the person you are asking has some experience in the matter, i.e. they have lost golf balls before.
That's what I thought when I first read the sentences, however other people may have differing opinions.