I tried to install Lion on my Mac Mini 2009. After downloading the i stallation file, I launched it (I also created a backup DVD beforehand). The first part of the installation took 3 minutes then it restarted the computer. However, since then, my computer refuses to restart. The screen stays dark (although the computer is on). After leaving the computer for 30 minutes, I did a forced shutdown (with the power button) and I disconnected the external hard disk and network cable. Then I restarted again. But the screen is still dark and I cannot hear any noticeable hard disk activity. What should I do?