If my mother passed when I was 16 and my grandparents on her side came down and sold everything in our old house, what are my rights to the proceeds?

Sorry to hear about your mother.

If your mother left a Will, the provisions of the Will are controlling as to disposition of her property.  For example, if your mother's Will left her property to your grandparents, then they were entitled to the proceeds from the sale.

If your mother did NOT leave a Will, the rules of intestate succession determine inheritance.  Intestate means dying without a Will.  Under intestate succession, if there was a surviving spouse, he would inherit your mother's entire estate.  If there wasn't a surviving spouse, you and your brothers and sisters would inherit equal portions of your mother's estate.  If there were any children, who predeceased your mother and left children (your mother's grandchildren), they would inherit the portion of the estate that their deceased parent would have inherited had that parent survived.

If there wasin't any Will, you and if you have any brothers and sisters would have inherited equal portions of your mother's property.  Your grandparents would not have inherited anything.  You can sue your grandparents and seek a constructive trust to recover the proceeds from sale of your mother's property.  A constructive trust means that your grandparents would be required to return the property or proceeds from the sale to your mother's estate so that your mother's children would receive their inheritance of your mother's property.