More than likely Nokia just wants to sell you a new phone. I would guess that a fairly cheap part such as the battery needs replaced, or possible the usb charging port but not the motherboard. These phone manufactures want you to buy new not fix what you have.
Lithium ion batteries are great but they will not last too long without proper care. You should always use the correct charger for a battery, while most wall and usb chargers will charge your battery, some provide the battery with to much current and some with not enough which in time can cause the battery to fail, and not hold the proper charge. You may have a 3.7v or 3.8 li-ion battery, but if the battery voltage goes below a certain voltage(usually under 2.75) the battery will put itself into a protective mode, which most people would assume its dead. Also a battery which is rated at 3.7v usually won't power on the device once it drops below 3.4v or less.
Unless water damage is involved I would guess the motherboard wasn't the problem.