Sony Ericsson K850i — Service Center promises to replace the phone

Hi,
I am Viddhesh Borkar from India - Goa. My contact number is [protected]. I have a problem with Sony Ericsson K850i. The problem is stated below.
I answered a call, and hung it up. After that, I put down my phone and noticed in my peripheral vision that the lights on my phone didn't turn off, so i looked closer and noticed that the backlight for the keys are ON, and the blue light surrounding the lens is ON, however the screen is dead, and the phone doesn't respond.
What is the cause for above problem ?

I already given my cell at Sony Ericsson Service Center at
Accel Frontline Limited
2nd Floor, Gomantak Bhavan,
St-Inez
403001 Panjim
They have told me that my cell will be replaced which means that i will get new circuit board fitted in old cell as they will be sending it to Chennai. Is this the meaning of replacement ? I think i am been cheated like this. Please can you reply me as this is a urgent issue. I have inveseted Rupees 20300/- for K850i SE Phone. I THOUGHT I WILL BE GETTING A NEW SEPERATE PHONE from the company.

Also please let me know about the warranty for the replaced cell ? I mean, will it be from the day i bought a new mobile or from the day i will get a replaced mobile ?
Thanks