When is the last time it successfully booted? It could be any number of things with a laptop that age. Could be something as simple as a bad PRAM battery or as complex as a bad hard drive. To start with, hold down CMD and R on startup to get into Recovery Mode. That will give you access to Disk Utility to check the hard drive health. Can also try holding down CMD+OPTION+P+R on startup to reset PRAM, although if it's a bad battery that may not help.