I accidently logged off with Guest user and now I cant get back into my computer it is threatening to loose all my documents

how to get my password back

Your Mac encrypts your password as it is entered. It is never stored "in the clear". It cannot tell you what it is, or even how many characters are in it. All it can tell is, when you enter it again and it encrypts it again, "was that it?"

So you will need to Reset your password, which changes it permanently, deleting the old one.

These article is the starting place. follow the Links in the articles for less-frequently-used procedures:

OS X: Changing or resetting an account password (Snow Leopard and earlier)

Reset the user password in OS X Lion, Mountain Lion and Mavericks

In addition, the guest login is intended as a temporary login that casual visitors can use without having access to anyone else's files.

When anyone logs into the guest area, any files they create will be removed when that user logs out or the computer is switched off. Any files belonging to other users (ie, the files in your original home folder) are unaffected.

So if you have accidentally logged into the guest area, and have created important files while there, you should preserve them - perhaps by copying them to an external drive or thumb drive, or by emailing them to yourself.

If you switched the computer off completely, then any guest files will already have been erased.

When you reset the password using the method that Grant describes, you might just like to reset it to a blank password so that you don't have to remember it.

Matt