Do I have a case if I invested in a business and I feel the people didn't do their due diligence and let the idea fail?

Probably not.  From the little you've described, these people probably had a fiduciary obligation to do their best to make the business a success, and if they really screwed up you might have a claim against them, but it is going to be very hard to prove either that (a) they truly acted negligently, particularly since (I assume) it would have been in their best interestsmas well as yours, to make the business a success; or (b) that the business would have been successful (and to show the amount you were damaged) if they had acted properly.