There are a lot of follow-on questions that could impact the answer. How long ago did your grandmother pass away? Has the will been probated? Is the executor a relative / heir of your grandmother? Have they given you a reason why it hasn't been sold per the instructions of the will? Is the house listed? Is the executor paying rent to the estate?
A lot also depends upon your relationship to the executor. I'd start the simple way - mail the executor a letter and ask for an update on the sale. If that doesn't get an acceptable response, then I would contact an attorney and ask him/her to review the will and send a demand letter on your behalf. Finally, I'd retain the attorney to challenge the matter in the probate court.
Based upon your answers to the questions above, those suggestions could change though. I recommend you take the will to an attorney and ask for a consultation. Many attorneys will meet with you for a free 1/2 hour to 1 hour consultation to determine the merits of your case.