Hi Everyone- some of you have worked it out already- but yesterday several of us were going back and forth on another thread, and I eventually contacted apple. I'm just going to copy and paste what finally worked for all of us. Use it, don't use it, share it- up to you.
If you've made it to the status bar with the black screen and the Apple Logo (after going through the white installation page) then Yosemite has installed- its just having problems opening. Why you ask? Because of something on start-up or a third-party program. So how do you go about fixing this? Follow the steps below:
1). Hit the power button to turn your computer off
2). Hit the power button on again - hit shift right away and hold it for approx 5 seconds and let go (this will take you into Safe Mode). It may take you about 5-7 minutes to load into safe mode, this is normal
3) Under System Preferences- on the bottom menu if you have a Citrix file sharing thing on the bottom remove it. (You don't need to remove Citrix the program just the application in the system preferences)
4) Check on the third-party non-Apple programs that you have. Check to see if they are up to date. Depends on the program how you do this.
5) If you have McAfee or another antivirus remove it (I know this sounds bad- but trust me its causing loads of people not to be able to launch Yosemite).
6). When in safe mode there is a variety of things that you need to do to make sure that Yosemite will boot. First you need to go into System Preferences -> Users & Groups -> Login Items. If there is anything at all in this list what you need to do is highlight it and click the minus sign below the list. Have nothing launch at Startup. There are also some files hidden away that have to do with start-up as well. When you click on the finder menu bar choose go... then choose computer. From there there are a couple of files you need to look at. Click on your hard drive... then on library the look to see if the "startupitems" folder is empty- if you've removed everything from start-up it should be empty and this is what you want. Then look at "LaunchDaemons" file under library- there can be things in here, but if you've removed the program then make sure the startup files are deleted here as well. Then check the "LaunchAgent" file- same thing as the Daemons file needs to be done. Then go through Go -> Computer -> Your Harddrive -> System -> Library and the same 3 files as before need the same thing done to them. Important- don't remove anything from LaunchDaemons or LaunchAgent that you actually have and use.
Like I said earlier if you are on the black screen with the apple and the bar without the countdown than Yosemite is installed. You'll notice when you're in Safe Mode that its actually Yosemite. Your problem is almost guaranteed to be a program causing it not to launch so after you've tried the above if it doesn't work you're going to have to try to figure out which program is causing the problem. From my experience the programs I know are ok are: Microsoft Office, Steam, Kindle, Spotify, Google, Keka, Firefox, Flixster, Microsoft Silverlight, Textwrangler, Citrix, Origin, Twitter, Dropbox (though don't have it on startup in any of the files), Skype, and Fotor.
After you've gone through everything above then either click start -> restart or start -> shutdown (if you pick shutdown you'll have to obviously turn your computer back on). After that it should go into Yosemite finally (if it doesn't- repeat and look for the problem program)