Try a few things:
- Reset PRAM. Reboot while holding down CMD-OPT-P-R keys until you see the system reboot twice and you hear a full volume bong sound. Do you still only see a white screen?
- Boot into safe mode. Reboot while holding the shift-key. Only white screen now?
- Reboot while holding down the OPT-key. Should see a screen showing all bootable devices. Select the device that is your OS. Do you still only see a white screen now?
i press shift and it boot into safe mode !!
and the imac it work in safe mode
when I log in my account it show me ( completing installation ... ) and I wait 5 min until the installation finished than I see my desktop finally but in safe mode
than I Reboot the imac again than it work finally!!!!!
knowing before The white screen happened I make update for the system from iTunes after that I reboot the imac than the white screen appeared to me
after that I tray to reinstall the OX and When it finished downloading the ox and install it than the imac reboot it self than it appear the white screen again!!
after that get into safe mode and it complete the installation than it work !!