My iMac does not fully start up and I have tried every keyboard start up code I can think of. When I press the power button it chimes that it has power and I see a white screen that turns black.

Try booting into Safe Mode. It will take a little longer to boot and the screen may flash or flicker due to the very basic video driver used in Safe Mode. It will run a file system check, delete some temp & cache files and loads only the minimal drivers. It also disables Startup & Login items. If it boots in Safe Mode, then try rebooting normally. If Safe Mode works, then it narrows down the problem.

What is the exact model of your computer? You can get this information by entering your serial number here. What version of macOS is on the computer?

Assuming you've used Time Machine to backup your system, you need to first try to figure out if you have a hard drive issue. You do this by holding down Command R (same time)when you turn it on and keep holding them down until you see the screen give you an option for recovery mode boot and restoring. If you need to restore from your Disk Utility, you choose your Time Machine HD and click restore from your last boot, or maybe go back a day or two in case you picked up malware that caused the issue. This will essentially erase and restore your HD (from your backup HD) to your last functional state. Please note, this process may be one of many for troubleshooting the problem and you may need to seek an expert to test key hardware components.