Why does Ubuntu, sometimes, fails to load?

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http://askubuntu.com – I recently installed Ubuntu 12.04 LTS on my SONY VAIO VPCEH (with Windows 7 Home Basic previously installed). Sometimes when I boot in Ubuntu, following things happen: Ubuntu loading screen seems to freeze. Esc key doesn't show installation progress. Hard-drive led doesn't glow. caps lock and scroll lock keys keep blinking. I've to manually restart my laptop. On restart, Ubuntu loads normally. What can I do to recover from present situation? (HowTos)