While rare, sometimes after updating OS X either with an upgrade or with an intermediate patch users have found that an account on the system may no longer be accessible. This may be because the system refuses to accept an account password, or because the account no longer appears. While such problems may have you initially consider reinstalling OS X and restoring from a backup, you may be able to fix the problem without doing this.
Missing accounts after an update
Local account information is stored in the system's directory database (located in the hidden /var/db/local/ folder), but if for some reason the database becomes corrupted then the system may lose the account entry. If this happens then the account will not appear in the "Accounts" system preferences or the log-in window; however, if you go to the /Macintosh HD/Users/ directory you will still see the user's home folder.
In this case the you can usually easily fix the problem by re-creating the account in the Accounts system preferences. Be sure you make the short username for the account the same as the existing home folder, and when you create the account the system will ask if you would like to use the existing home directory for this account. Doing this will re-create the directory entry for the account, and then link that directory entry to the existing folder so all that user's data and settings will be preserved.
When this is done, you might consider resetting the home folder permissions for the account, which can be done by booting to the OS X installation DVD and choosing the "Reset Password" tool from the "Utilities" menu (after selecting your language). With this tool open, select the account username and then select the option to reset home folder permissions and ACLs. This step may be necessary to ensure that the folder's security is intact, and that applications the user runs can properly access items in the user's library. Then restart to the main boot drive and log in to the account.
If the home folder for the account is missing and you have a backup for it, then you can copy the home directory from the backup to the /Users directory and then perform this procedure to create an account and link it to the restored home folder.
Incorrect password after an update
Another problem beside missing accounts is that an account's password may no longer work. In this event the fix is a lot easier, and just requires you to boot to the OS X installation DVD and then use the same "Password Reset" utility to change the password for an account (see this article for more information on resetting user passwords, and this article on security and data safety concerns about doing this).
If the password is reset to be the same as the old one then you should not need to do anything further; however, if you changed your password then your log-in keychain will become unlinked from your account and will no longer automatically unlock when you log in. To fix this, after changing the password and logging in, go to the Keychain Access utility, right-click the log-in keychain, and change the password for it. You will need to enter the former account password in the "Current Password" field, followed by the new password in the corresponding fields.