Yosemite downloaded and now gray screen

I've shut down the ciomputer numerous times and restarted it and nothing indicates an estimated time remaining. Here's what happens:

The computer asks me to select Laurie or guest. I select Laurie.

I enter my password

Then, a bar (progress bar??) looks like it's downloading something. It stops about 1/4 of the way through.

Then, I get a dark screen.

Then, the spinning ball.

Then, the cursor appears.

Then, it looks like the desk top is trying to load.

Then, it asks for my password again.

I try to enter it, get the ball again.

Then, it lets me enter the password

I click the arrow to execute

Then, the computer freezes

About 5 minutes later, the desk top screen loads the toolbar and wallpaper, but none of my icons come up.

I click Safari in the tool bar and nothing happens.

I tried all of your suggestions. The screen that is supposed to appear when hitting the shift key or the command s key, pops up briefly and then goes away, leaving the login screen. I appreciate your efforts. Time to go to the genius bar...

Laurie