IMac sticking at the apple logo screen

Question

I just turned on my 24 inch iMac  it starts to boot, the music plays, the apple logo appears then the whirley wheel appears and the machine sticks with a blank grey screen. I have tried Turing it on and off. I tried plugging in a wired keyboard in place of the wireless keyboard and pressing keys at random, but no change.
The iMac is  running Lion and has been updated. It's been behaving.
Anyone have any suggestions
Andrew

Answer 1

If you have more than one user account, these instructions must be carried out as an administrator.
Triple-click anywhere in the line below to select it:
syslog -k Sender kernel -k Message CSeq "I/O error" | tail | open -f -a TextEdit
Copy the selected text to the Clipboard (command-C).
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window (command-V).
The command may take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear.
A TextEdit window will open with the output of the command. Normally the window will be empty. If you get output like this:
kernel[0] <Debug>: disk0s2: I/O error
the boot drive is failing, or there's some other hardware fault in the storage subsystem.