LG Electronics — lg monitor display problem within warranty period

Address:

North 24 Parganas, West Bengal, 700056

I have purched a lg monitor, this monitor is in 3 years warranty period, i have all documents but i lost my bill.
Manufacturing date & barcode is attached in the monitor, i was contact customer care & also service centre but they are denie it
Because of my lost bill but legally they can't denie, pls take the matter as early as possible