When updating iTunes to the most recent version, my iMac froze. No keys worked; could not force quit so I turned off the power. Now when I turn on the machine I get a white screen. Any ideas on what I can do to get my machine running?

I am unable to provide basic information on the system because I am unable to access anything. The iMac is about six years old. The software was current. I was updating iTunes to the latest version when everything stopped working. The various methods to force quit the operation do not work. I was forced to turn the power off, but now when I reboot I only get a blank screen. (The Apple logo will appear for a few seconds before going all white. )Everything is backed up with Time Machine, but again I can not access it from this computer. Any ideas on what I need to do to get the machine running? Thank you..