Hypothesis incorrectly stated and submitted
I'm hoping to be correct here, but this depends partially on personalities. Since you found the error, you should immediately inform everyone involved of the error and offer to supply a corrected version immediately. If people are reasonable they should act reasonably, of course. If you wait, and someone else notices it, you will have difficulties. 

Getting things absolutely backwards is likely more common than you imagine. Also, when you write and then proofread your own writing, you are very likely to see what you want to see, what you think you wrote, rather than what you actually wrote. Most reasonable people will recognize that, have a bit of a chuckle and move on. 

But you have to be the one to raise the flag. 

You raise this issue on a week end. If at all possible, don't wait for the new week to inform at least your professor. 



One of the reasons that books have editors and journals assign reviewers is to catch these kind of errors of inattention. Sometimes an advisor will perform that function for a student, but not always.