iMac update to 10.6.7 - blue screen on restart
I've updated my iMac (27 inches end of 2009) yesterday with 10.6.7. using the combo updater. When it restarted, it went through the grey screen with the Apple and got stuck afterwards at the blue screen where it seemed to be stuck in an infinite loop for half an hour (cursor disappearing, blue screen slightly dimmed then cursor reappearing...).
I restarted from the installation disk and restored from the last available TimeMachine backup (10.6.6), but with the same result. Grey screen then stuck at the blue screen where the cursor appears then disappears.
I then restored a previous backup but with still the same issue.
Any clues to what I can do next?
Many thanks.
Raf
iMac 27" end of 2009, Mac OS X (10.6.6)