I've seen a question on which a bounty has just been set (the question is this). After a quick look (following the two comments) I saw that the question appeared after the owner simply edited the old question into something new and totally unrelated. How should this be handled (if it's not an accepted situation)? Can I simply answer the question or should I flag it?
The answer is the same as if the question didn't have the bounty.
Assume that the question has no answers, is it acceptable?
I would say it depends on the votes:
Either way, a new question should be asked, even if you have zero answers (and I would even say zero votes).
I say you flag it for moderator attention (it has been, and been dealt with).
Note in this case the bounty wasn't removed because the content completely changed, but because the question is close-worthy, and the question can't be closed without removing the bounty.