Q: why can't I log into my mac after yosemite

Try restarting holding the Command and R keys.  From the Utilities drop down select Disk Utility, choose the internal drive and then Repair Permissions.  Go to the apple left side of the menu bar and Restart.

If that does not fix it, repeat the restart on the recovery HD, but this time click Utilities to get Terminal.  When Terminal opens, at the cursor type exactly:

resetpassword

And press Enter.  When the Resetpassword window opens, highlight the drive, choose the user ID and type the password twice, leaving the Hint blank.  Save and accept the next dialog that opens.  Lower right of the window agree to reset the user home directory permissions and then restart again.  See if that clears the problem.

You're using the "legacy" (pre-Lion) version of FileVault, and the disk image file on which your home folder is stored is corrupt.

This condition is quite difficult and time-consuming to recover from. If the procedure suggested here is too hard, get help from someone more experienced. One option is to make a "Genius" appointment at an Apple Store. Legacy FileVault is obsolete and should not be used under OS X 10.7 or later.

If you have another user account with administrator status and can do so, log in to it and skip to Step 4.

Step 1

Start up in Recovery mode. The OS X Utilities screen will appear.

Step 2

Select

          Utilities ▹ Terminal

from the menu bar. In the window that opens, type this:

res

Press the tab key. The partial command you typed will automatically be completed to this:

resetpassword

Press return. A Reset Password window opens. Select your startup volume if not already selected. Pull down the menu labeled Select the user account and select

          System Administrator (root)

Follow the prompts to set a password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9.

Select

           ▹ Restart

from the menu bar.

Step 3

At the login screen, click Other... Enter "root" (without the quotes) in the Name field, and enter the password you set in Step 4 in the Password field. You should now be logged in. Do nothing while logged in as root except as indicated below.

Open the Users & Groups preference pane. Create a new user and check the box marked

          Allow user to administer this computer

Restart and log in as the new user.

Step 4

Load this web page in Safari.

Triple-click anywhere in the line below on this page to select it:

sudo dscl . -delete users/fubar HomeDirectory

Copy the selected text to the Clipboard by pressing the key combination command-C.

Launch the TextEdit application. Paste into the window by pressing command-V. Substitute the short name of the affected FileVault user for the word "fubar".

Triple-click the edited line and again copy to the Clipboard.

Launch the built-in Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.

Paste into the Terminal window. When prompted, enter the password of the user you're logged in as. Nothing will be displayed when you type it. Type carefully and then press return.

Step 5

You should now be able to log out and log in as the FileVault user. Your files still won't be available. FileVault is inactive.

Locate the disk image file named "fubar.sparsebundle" or "fubar.sparseimage" at the top level of your home folder. Again, substitute the short user name for "fubar".

Step 6

Try to open the file. If it won't open, try to repair it in Disk Utility. If it still won't open, restore it from your most recent backup. Keep trying older backups until you find one that works. If you don't have a backup, the data is lost, perhaps beyond any hope of recovery. There's a slight chance that you might be able to repair the disk image using an expensive third-party application called "Disk Warrior." If possible, go to a "Genius Bar" where the software is available for use by the "Geniuses" at no charge.

Step 7

Copy the files from the mounted disk image to the corresponding subfolders of your home folder. The Library folder is invisible in the Finder. Open it by holding down the option key and selecting

          Go ▹ Library

from the Finder menu bar. Open the copy on the disk image by selecting

          Go ▹ Go to Folder

and entering

/Volumes/fubar/Library

in the text box that opens. Again, substitute the correct name for "fubar".

Log out and log back in.

If there isn't enough disk space available to complete this step, unmount the disk image, copy the disk image file to an external storage device, delete the original, and try again.

Step 8

Back up all data, then delete the disk image file.

Step 9

Delete the user you created in Step 3, if applicable.

Step 10 (optional, but recommended)

Follow the instructions in this support article under the heading "Disable the root user."

What happens when you try?

As I wrote, "fubar" is just an example. You have to substitute the name of the affected user.

Either you didn't enter the right command, or legacy FileVault isn't enabled. The user name you need to enter is the short name, which is usually different from the long name.

Do you know whether legacy FileVault was enabled?

While logged in as the new user, delete the old user, choosing to keep the home folder, and recreate it with the same name and password.