If it's the same battery when you first got the phone then change it as batteries do deteriate after a while and they don't give any warning they are losing power, they are like a woman abrupt, and switch off.
Take it in to a shop and they may be able to tell you what state the battery is in but again my guess is it needs a new one, my Nokia did after two years that wouldn't hold its charge for long.
BOB, February 2005
