Recent Password enforced to log on Update

[stevef]

As a result of installing an update a handful of days ago, I now have to use a password to log into my account

That's a bit odd. Do you have any applications installed that might do something like this ?

It is typical that logging on to the desktop requires a valid Username/password combination for security reasons.
For convenience, it is possible to set Linux Lite to 'enable auto login' to remove the requirements to enter user credentials so it may be worth trying this.

Click Menu
Begin to type in "Lite Auto Login"
When you see the option for Lite Auto Login offered click on it
Click on 'Enable Auto Login'
You will need to enter your user password here as it is a system change.
Read and the warning and if you want to go ahead click the Enable button.
There should be a message that the change has been made.
Close the chooser window and restart.

Would any of you know if there's a default password for non-admin users?

Don't think there is - it would be a very bad thing if there was.

please tell me how without the use of another computer I can reinstall Linux light.

That is possible but is drastic unless there is something else wrong.  The help manual describes installation.