Xiaomi India — Service centre return a different phone

Address:

700051

On 14 th june 2016 i submitted my mi 4 i phone to their service centre in kolkata sec 5. After 45 days they returned me a mi4 i phone but imei nunber was different plus mi account is registered with a different person. Therefore all my data initially got synced to that acoount.
This is a pure case of fraud.
If by any how my data get lost ot that person who used to be the actualy owber of the phone comes from a criminal background then it can create a great trouble for me. I urge you to help me on priority.