1. Boot up your system and select Recovery Mode from your boot menu.
2. Login by typing
at the login prompt and then type in the root password.
3. At the hash (#) prompt type
4. Watch the screen carefully for any error messages or information of any kind.
5. Assuming that X won't start (no gui appears asking for your own userid and password) type
6. Scroll down the file quickly looking for any lines beginning with (EE) which means "error".
7. Report back the errors that you have found.
I hope that's step-by-step enough for you.