Help!

I have a Toshiba Satellite L555 running Windows 7 Home Premium. When using the computer about 30-40 minutes while using it the screen freezes up and never unfreezes unless you restart the computer and then it just does the same thing again.

I need help. I dont know why it is doing this. I was told it could be the hard drive so I replaced it and it is still not fixed.

Any ideas? Please HELP!

-Blaine