Problem: The desktop will fail to launch sometimes after login. It'll just present me with a completely black screen. When this happens, I'm forced to hold the power button and shut down the machine and restart it.
When you will start Elementary OS, before you will login, I mean before you will type login / username and password, press combination of ctrl+alt+F1 to F6, usually I press F4. It will show you the terminal. Login there, I mean type your login and password, then type this in terminal:
sudo mv /etc/xdg/autostart/at-spi-dbus-bus.desktop /etc/xdg/autostart/at-spi-dbus-bus.disabled.desktop
It will ask for the password, which is actually the password you are using to login.
Then press ctr+alt+F7 to go back to GUI. Login and let me know did it solve the issue permanently.
You can reboot few times to check it.
What is funny, is the fact that this is really old bug.