I cannot login to my mac.

Every time I have opened it and turned it on it goes to the white apple loading screen. After about 30 seconds - 2 minutes it just shuts down again. This began after a software update for things like word, quicktime, etc... when the computer "restarts" after an update. It never came back on. Help! I replaced the battery within the past 6 months, but it is a 2008 macbook.