My new TB48 battery has inflated. No reason for it since it's new and charged only few times (less than 10). I can't even turn it on.
What sould I do?

It is under warranty, please contact the support center http://www.dji.com/support
The only was a battery should swell up if stored fully charged for a month or more.
But that cannot happen with the DJI intelligent batteries because the storage charge default is 10 days meaning if left alone charged, in 10 days it will start the discharge process.
If you are storing a battery, DO NOT press the power button because even a short press will reset the timer.