You have accepted their offer, negotiation is over. Accept this. Now, you have a decision to make -- do you want the 15k or do you want the company? Either is a reasonable choice.
If you want the money, politely inform the company that another opportunity has arisen and you can't pass it up. Thank them for the offer, apologize for any inconvenience, be upbeat -- they aren't bad, you simply have something better. Do this as soon as possible.
Don't go into the details about how this offer is better unless they explicitly ask. Do NOT hint that you want to renegotiate.
If you want the company, simply decline the offer.
If you're wondering about my emphasis or how this differs from the other answers: In work, you trade time for money and the company trades money for time...
So, when you accepted the offer, you said "this is the price you have to pay for my time", renegotiating is unprofessional and dishonest because without any change in what you are offering, you are trying to change the price.
But while you don't have any more to offer them, you have (depending upon what you find more desirable) found someone else that values your time more. This is a change not in what you have to offer, but in your circumstances -- and it iscompletely independent of the original company, it has nothing to do with them. But it does impact your life, just as it would if your spouse was transferred, or your child was accepted into a really good school.
It's entirely reasonable (and should be expected) for you to reconsider your situation based on your circumstances, and then to decide in a manner that you believe is best for you.