My iMac does not boot up....gives me an error message...can anybody help?

Do you have installer disks with it, if yes startup from the installer disk, then when the installer is there go into the menu bar on top and choose Disk Utilities, click on your HD and repair permissions and the disk, when finished start normally again, now see if it resolved your problem.

 

This is a "Kernel Panic".

The cause could be many different things. Could be bad RAM, or another defective component.

In the case of my own iMac G5, I had this happen because of a bad network card. Once I removed the card, all was fine.

 

^^Probably no HW failure here, the OP shut it off manually, now this can cause file corruption, likely in the file structure and can be repaired.

 

As I said in my former Post, put the installer disk inside then shut it down, after this hold the C key when you start up, it should now startup from the CD/DVD, when the installer launched you'll see the menu Bar on top, click on Utilities(pretty sure it is called that way) and select Disk Utilities, it will open, when it is open click on your Hard disk on the left, on the right you'll see 4 buttons, two of them are repair permissions and repair disk, do one at a time, when finished close Disk Utilities, installer will appear again but go back to the menu and click restart.