Updated latest security software as prompted, now Mac won't restart....

Question

The screen is up and running the little 'wait' thingy, for the last 12 hours. I don't want to turn it off and on manually until I know more, as my IMac crashed for no known reason once before and it was many dozens of hours of searching and experimenting before someone was able to finally get it back on. It was a software and not hardware problem, the store tech ascertained.
It has been quite slow since it was fixed, but working normally otherwise. The time I last turned it on, that morning, it started normally, but just starting Entourage took 2 full minutes, timed on a clock. The update alert came up and I chose to update just the security one. I didn't restart immediately as I had other programs up. Later when I was done with them, I clicked on the restart button on the update screen, and then it just wouldn't start up again, as described above. Is it something to do with the update?
I don't know why a powerful computer would run slowly in the first place, but this not starting happened with the security update's required restart.
New here, and I thank you for any help.
Natalia

Answer 1

Your only solution may be do do a hard shutdown, press the power button until the machine powers off, then :
Disk Utility
1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Click Repair. Disk Utility checks and repairs the disk.
If no problems are found, start up normally, and please post back with what OS you are running and what the update was, but don't install it again just yet.
If DU finds problems it can't repair, then I'm afraid, unless you have DiskWarrior, you will need to take it back to the repair shop.