Well first, we would need to know if you're in Brooklyn, NY or Seattle, WA, and then we would also need to know what contract forms you are using. I'd suggest it would be better to simply contact an attorney because there is a need to review the forms and get a lot more facts than what a two sentence post provices.
That said, as a general rule: "There is no late in real estate." Thus, assuming the seller was not somehow at fault for the inability to close, they could probably ask for whatever they wanted for an extension or pursue their remedies for breach. If it is their fault it would be much more complicated because you'd be dealing with buyer remedies for a seller breach. Which gets back to the attorney comment.