One page gone for Index

[pemba76]

As i said before that i have a website that had just 3 pages in the main index:
www.example.com
www.example.com/a.php
www.example.com/b.php

But today i saw that www.example.com/a.php has totally disappeared from g results. Even when i type www.example.com/a.php in the search box in g, it shows "Sorry, no information is available for the URL".
The other url's is showing properly.
What can be the reason for this? Please comment!