Q: Mac stuck on white screen after software update

I updated to snow leopard and then then to 10.6.8. After the updates had installed, my computer restarted and now it's stuck on a white screen. No key shortcuts work at all, I've tried. None of them work. Every time I try to turn it on it's just frozen on a blank white screen. Twice it's gone to the apple logo and been able to start up after 20 or so minutes of being on the white screen but each time it froze. I don't know what to do.