While on my Personal computer (Windows XP), the monitor went blank, I heard a click from the personal, then the Windows start page appeared (where it asks for password).

On entering the password, the screen then showed what was on it the moment before it went blank.

This has not happened before, I’ve not changed any power settings. Any ideas what happened?

Answer:
Your hard drive might be faulty if you can check device manager for any exclation marks or question marks and download hdtune it tells you the status of the hard drive how long has it been doing this.
Answer:
We’d a milk monitor at school just the same. He went blank each time you asked him for his dinner money, we just re-booted him. Cry baby.