You have made numerous mistakes in this project and are now paying for it.
First, you should never, ever work 14 hours a day continuously. You can do that kind of crunch for one or two days in an emergency, but if you think you can keep that up, you have absolutely every piece of research ever done on the matter against you.
Second, you should have brought this up as a problem much earlier, before tiredness and burnout become visible to the client.
Third, you should not have given up so easily on the added workload. You should have stood your ground and made it clear that you can not do this work in considerably less than the estimate you already gave. If the manager thinks you can, you should sit down with him and put your estimate and reasons on the table and he his. It is common for managers to make estimates for things they know little about, and it is necessary to push back on that when it is completely wrong - your manager needs feedback to improve his estimation skills.
Fourth, you did not see that this would be coming, but it was a virtual certainty from the way your describe the situation.
What to do:
First, you need every piece of paper or mail that supports your position gathered in a safe place (e.g. export the mails to an encrypted folder or something). You don't know how foul people might play with you. Take care to not violate any company rules, especially about putting data on non-company storage, but having a second copy in a non-obvious place (within the company infrastructure) can't hurt.
Second, you need to answer to those accusations. And you need to point to your warnings and how they were ignored.
Third, you need a fine ear on how this is going to be played. How well do you know your manager? Is he more likely to do a constructive approach and try to solve the problem, or is he more likely to make you take the fall so his record remains clean? Your actions depend entirely on this. If your boss is setting you up as the scapegoat, you can leave or destroy him.
Fourth, you need to know your position and how strong you are. I've been in a somewhat similar situation before (not the same, but being blamed for what was bad management planning). I knew I was in a strong position, so I handed in my resignation - and almost immediately got a personal offer from the CEO for a different position in the company.
So in short, you need to know your cards and the rules of the game being played. There's not enough information in your question to recommend a precise course of action, but covering your ass, being ready for a fight (or a quick exit) and understanding what you did wrong - and being ready to admit it in the right moment, to the right person and nowhere else will be the key ingredients.