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Back up all data now.

This procedure will unlock all your user files (not system files) and reset their ownership and access-control lists to the default. If you've set special values for those attributes on any of your files, they will be reverted. In that case, either stop here, or be prepared to recreate the settings if necessary. If none of this is meaningful to you, you don't need to worry about it.

Step 1

If you have more than one user account, and the one in question is not an administrator account, then temporarily promote it to administrator status in the Users & Groups preference pane. You can demote it back to standard status when this step has been completed.

Drag or copy — do not type — the following line into the Terminal window, then press return:

sudo chflags -R nouchg,nouappnd ~ $TMPDIR.. ; sudo chown -R $UID:20 ~ $_ ; chmod -R -N ~ $_ 2> /dev/null

Be sure to select the whole line by triple-clicking anywhere in it. You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up. You don't need to post the warning. If you don’t have a login password, you’ll need to set one before you can run the command.

The command will take a noticeable amount of time to run. Wait for a new line ending in a dollar sign (“$”) to appear, then quit Terminal.

Step 2

Boot into Recovery by holding down the key combination command-R at startup. Release the keys when you see a gray screen with a spinning dial.

When the OS X Utilities screen appears, select Utilities ▹ Terminal from the menu bar. A text window opens.

In the Terminal window, type this:

resetpassword

That's one word with no spaces. Then press return. A Reset Password window opens. You’re not going to reset a password.

Select your boot volume ("Macintosh HD," unless you gave it a different name) if not already selected.

Select your username from the menu labeled Select the user account if not already selected.

Under Reset Home Directory Permissions and ACLs, click the Reset button.

Select  ▹ Restart from the menu bar.

To be clear, any files that are password protected (for instance) will lose such protection and will need to be redone? Is that what you mean?

No. Any files with permissions, ACL's, or immutable flags that you have set will be reverted. If you don't understand what that means, then you haven't done any of those things intentionally and you don't need to concern yourself with it.