Hai,
My site has been cached by google on Jul 22 and after that googl didnt index my site.

I have done changes to the title ans Meta tags and expecting changes in ranks for few keywords. But as Google didnt index my site, after the changes I did, the ranks for the keywords are same.

How to make Google to index the pages, when I do any modifications to the site?

thanks.

As your site once indexed by google, now all you need to do some off page optimization for your site by doing some link building and directory submission. Also check for some important issues with your site that google dont like for the new sites. Like
cannonical issue, duplicate pages or content, improper redirections, bad neighbourhood.

This will definitely get back your site in Google, ranking of the keywords will come only after proper indexing.

all the very best...

Hai,
My site has been cached by google on Jul 22 and after that googl didnt index my site.

I have done changes to the title ans Meta tags and expecting changes in ranks for few keywords. But as Google didnt index my site, after the changes I did, the ranks for the keywords are same.

How to make Google to index the pages, when I do any modifications to the site?

thanks.

Does your server support "If-Modified-Since"?

If not already done, you should get a webmaster tools account and submit a sitemap.

https://www.google.com/webmasters/tools/docs/en/about.html

...and try to get some other sites to link to you.