Pain in molar that has an old root canal and old crown, what could the problem be?

I have a molar that has both a root canal and a crown, done at least 15 years ago. All of a sudden yesterday I started having pain when biting on that tooth. If I don't chew on that side, there is no aching or throbbing. Sleeping is difficult because I find that I do bite during my sleep and it wakes up every time now. I will see my dentist next week if he has an opening. What could this be?