Mac book pro won't let me login

What exactly do you see and what happens?

Please read this whole message before doing anything.

This procedure is a diagnostic test. It’s unlikely to solve your problem. Don’t be disappointed when you find that nothing has changed after you complete it.

The purpose of the test is to determine whether the problem is caused by third-party software that loads automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you turn on the computer, and again when you log in.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to start up and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup may also be somewhat slow.

The login screen appears even if you usually login automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, restart as usual (not in safe mode) and verify that you still have the problem. Post the results of the test.

The easiest way by far to recover is to restore the startup volume from a backup that predates the issue. If you can't do that or don't want to, the following procedure works sometimes. There's no assurance that it will work in your case. It's only suitable for advanced users. I can't offer instructions any more detailed than those below.

Back up all data if you haven’t already done so. There are ways to back up, even if you can’t log in. Ask for guidance if you need it. Do not attempt this procedure unless you have a current backup of all data.

Briefly, you need to rename the following folder:

/Library/Preferences

If you already know how to do that, you can skip the rest of this message. Do it, and try again to log in.

Step 1

Start up in single-user mode by holding down the key combination command-S at the startup chime. If you’re using an external USB keyboard, it must be plugged directly into a built-in port on the Mac, not into a hub. Release the keys when you see a black screen with scrolling white text. In this mode, there’s no graphical interface; just a command line. Initially, you can’t make any changes to files.

Step 2

When the text stops scrolling, and you see a line ending in a pound sign (“#”), type the following command:

mount -uw /

You must type the command exactly as given, with no mistakes.

Press the return key.

Step 3

Enter the following command:

mv /L*/Preferences /Library/Preferences.old

Again, you must get it exactly right.

Step 4

Enter the command:

exit

Press return. The text will start scrolling again, and then the system will restart. Try again to log in. If successful, you can experiment with restoring some of the contents of the Preferences folder from a backup, or if necessary, from the renamed folder Preferences.old, although you may have trouble with permissions if you do the latter. Otherwise, just recreate all system-wide settings and delete Preferences.old by dragging it to the Trash.

Thank you so much for posting this! Using the pad was letting me move the mouse pointer however the clicking function wasn't working. I thought I had lost everything... plugging in a remote mouse worked!