Late 2009 iMac won't start up after Yosemite update

[michelle.m]

Hi there, I am new to Mac Forums, but this morning I tried to update my iMac (late 2009) to Yosemite, and it installed and restarted to the login in screen. But then the wheel of death showed up when I tried to click on my account to login. So I restarted the computer and now a white screen shows up with the Apple and then a circle with a dash through it appears where the apple was and the computer won't do anything?? I've tried to restart and unplug and Safe Boot, to no avail.
Help please! Thanks.

 

[chscag]

Welcome to the Mac Forums Michelle!

Did you make a backup before you installed Yosemite? If you did, restore from backup and then try again to install Yosemite. If you have no backup, you may be forced to reinstall and could possibly lose data. Let us know and we can go from there.

 

[michelle.m]

I did not make a back up. Fortunately, I don't store much data on this iMac, I mostly use it as a "thunderbolt display" for my Macbook Pro.
So, without a back up, is there any way I can reinstall?

 

[chscag]

Yes, you can reinstall without a backup. Restart the iMac and hold down the command and r keys at the same time. Keep holding them until you see the Apple. That should get you into recovery. From the recovery partition you can reinstall Yosemite. I guess I should ask... what version of OS X were you running before attempting to update to Yosemite?

 

[harryb2448]

G'day and welcome to the forums.

Still like that Michelle?

This was normal during Yosemite beta testing. Turn off your iMac using the power button on the rear. Hold down for about ten seconds until it turns off. Disconnect mains plug and leave it for five minutes. Connect back up, turn mains on and hit the on/off button and hospefully it should start.

Keep us posted on the result?