Black screen after boot

Guys I really want to use ubuntu but i have a big problem. i've successfully downloaded and installed ubuntu on my pc (desktop edition). Then after installing, i restarted my pc. The logo appears during boot and also the sound but then after that my screen went black. my mouse pointer is moving though but nothing happens. Need help please.

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Try

Try pressing CTRL+ALT+F1 or F2,F3,F4 Does it open a text console with a login prompt?If so login and type dmesg or  dmesg | more and see  if you can find any error messages. You can also look type more /var/log/Xorg.0.log to see errors related to the X-server at startup. Anyone can play with the consols with the above keys return to graphics with CTRL+ALT+F7