It is still a valid contract but there could be a considerable delay before you are able to move into your house. The property forms part of her assets and her deceased'd Executors will have to calculate what all her assets are worth before the Will can go forward to Probate. If the deceased had assets above the Inheritance tax level, the executors will also have to pay the inheritance tax due on the estate before any bequests and disposal of other assets can take place.
If such a delay would cause you major inconvenience, you may wish to contact the Executors, if you can find out who they are, to discuss whether they would be prepared to release you from the contract. I don't know whether this would be legally possible but it may be worth investigating.