If she dies without a will the laws of "intestacy" apply. Without getting too bogged down, this is what would basically happen (if you live in England):
If there are no children, grandchildren, or great-grandchildren then you inherit her entire estate. If there are children, you receive her personal possessions and the first £270,000 plus half of everything over that amount. The children receive the other half of that amount.
If she has made a will her estate is distributed according to that will. What happens to your joint property depends whether you are joint tenants or tenants-in-common. If you are joint tenants the entire property reverts to you (regardless of whether or not there is a will). If you are tenants-in-common you each own a discrete share of the property. Her share will be dealt with either under the rules of intestacy or in accordance with her will.