Disconnect all peripherals from your computer.
Boot from your install disc & run Repair Disk from the utility menu. To use the Install Mac OS X disc, insert the disc, and restart your computer while holding down the C key as it starts up.
Select your language.
Once on the desktop, select Utility in the menu bar.
Select Disk Utility.
Select the disk or volume in the list of disks and volumes, and then click First Aid.
Click Repair Disk.
Restart your computer when done.
Repair permissions after you reach the desktop-http://support.apple.com/kb/HT2963 and restart your computer.
Yesterday i downloaded driver and software for hp 8600 printer. Today, the imac 2009 does not boot. All i get is a white screen.
What happended is a incompatible software was installed or it's corrupt in some manner that OS X is not booting.
You need to replace OS X itself with a new copy
Run through this list of fixes, likely by #8 you'll be booting again normally.