I recently updated my 2 year old iMac over the internet and now it will not start up. When I press the start button the normal start up sound is heard and the apple logo appears on the screen followed by the start up sequences then the screen goes black

You may need to restore the Firmware.

You may want to try booting into the Recovery HD and checking the disk for problems.

Restart holding Command+R, when it boots (hopefully), select Disk Utility and then select your Macintsoh HD and click on Repair Disk. If any errors are found/corrected click on 'repair disk' again until you get a clean pass. Restart as normal from the Apple menu.

If this doesn't help, try a Safe Boot: restart holding down the shift key until you see a grey progress bar. Once booted (again, hopefully) restart as normal from the Apple menu.

The progress bar indicates that fsck is running. It has nothing to do with firmware updates.

This seems to have worked for me!

My 1996 iMac would not start after the last update, working now!!

Since you have the old hard drive, have you tried to see if the replacement Mac will recognize it? That would be very easy and relatively inexpensive. You can get an external enclosure from OWC, http://www.macsales.com for $20 - 30 and just drop the old drive in the enclosure, plug it into a USB port and see if the Mac can access it.

Recently removed the hard drive from a MacBook Pro my daughter had so she could use th drive as an external with a new MBP. Works great.

You're welcome. And if the drive really doesn't work, you are only out a small amount for the effort...if it does work, you have all your material you thought you had lost...so good luck and hope this works out for you.