2010 mac pro with OS X10.7.3 won't boot.
I shut down my mac yesterday and when I restarted it today the chime sounded as usual the white screen appeared and the apple logo along with the spinning disk underneath also appeared then the screen went bright white for a second as it uually does before the desktop appears then the spinning disk that appeared beneath the apple logo during the boot process reappeared and just keeps disappearing and reapearing.
I have tried to boot into safe mode and have also tried to boot onto my USB key that I used to install lion but the mac pro just ignores my key presses and goes through the process described above.
So I am not sure what to do now.