ubuntu : black screen on startup
i have a lenovo Y330 laptop with ati radeon graphics card. i have been running ubuntu on it for 1 month, and it's been working fine. however, since yesterday, whenever I boot up, it works fine , everything loads perfectly until it reaches the point where the login screen should be appearing. instead, i see a black screen, with only the cursor( the very same cursor which appears just before login) with a busy sign. im able to login to text mode. some people suggested me to change some stuff in the xorg.conf file, but it has been unsuccessful so far. please help me out..