MacBook Air 13 won't accept either user name or password. Don't recall ever happening before. Have tried every conceivable incorrect spelling etc. Even the Apple password clue prompt does not work. Can get into guest user ok.
User and password worked ok to get into this discussion board!
Another reason why your password might not be recognized is that the keyboard layout (input source) has been switched without your realizing it. At the login screen, you can cycle through the available layouts by pressing the key combination command-space or command-option-space. See this support article.
Otherwise*, boot into Recovery mode. When the OS X Utilities screen appears, select
Utilities ▹ Terminal
from the menu bar. In the window that opens, type this:
Press the tab key. The partial command you typed will automatically be completed to this:
Press return. A Reset Password window opens. Close the Terminal window to get it out of the way.
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.
Follow the prompts to reset the password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9.
from the menu bar.
You should now be able to log in with the new password, but your Keychain will be reset (empty.) If you've forgotten the Keychain password (which is ordinarily the same as your login password), there's no way to recover it.
*Note: If you've activated FileVault, this procedure doesn't apply. Follow instead these instructions.