Law will be concerned only with your relief as per the provisions of law and it will not go behind your issues of sentimental attachment to the lost property.
If you consider that the mangalsutra as sentimental, then you should have actually worn on your neck and not keeping it elsewhere.
The importance of mangalsutra is to identify the woman in the society as a married woman, hence if you had kept it in the locker there is neither a question of sentimental importance attached to it nor the purpose of wearing the manglasutra at the time of marriage has not been solved.
So you may decide whether to pursue the matter legally, if yes, you may follow the suggestions rendrered here or can consult a local lawyer and proceed as suggested.