Is it obligatory for one who neglected to pay zakat for years and then had her money stolen, to make up for the zakat of the past years, or does the theft serve as expiation for not paying it?

Whoever has withheld zakat must repent to Allah and calculate the zakat for all past years and pay it in full. It is a debt upon him that is not waived by the loss of wealth, as Allah Almighty says: "And whatever strikes you of disaster - it is because of what your [own] hands have earned." Perhaps the calamities that befell him are an expiation for his sins.