My macbook was stolen and locked by icloud.

My Macbook was stolen this Dec 16, 2015. Last night I received an email that my mac was online so i logged in to my icloud account. I found out that my name was changed and showed online. Then next email informed me that it was locked but i cannot detect the location. Please help, need to recover my mac because several work data was in there. At present it was offline and locked. Thank you for the immediate reply.