Nokia 5310 xpress music
Mobile going switched off automatically

I have buyed the nokia 5310 in dec from ranchi.now the mobile automatic switched off just after switching on the mobile.thats why i am suffering with lot of problem due to this mobile.now just tell me wht i have to do next and where it will repair.the mobile is under waranty period but somehow i have lost the reciept of the mobile..so give the suggestion & the address of hte service center where it will repair.