HTC mobile problem between me and company

I have a htc 816g phone. The screen broke of the phone because of over heating on its own (as what i feel) . The service centre personel took the phone for repair on 15th april 2015 and handed me the bill og 4100 approx. But since then phone has bot been rectified and neither service centre nor customer care are responding up to the mark. Every week they shift the delivery date by a week ahead. Please help me out. I neither got a stand by phone despite several reminders. I am now helpless. My phone is in warranty till nov 2015