Thank You Maxi..
Previously created were sitemap.xml, sitemap1.xml
Then we went according to Google guidelines If you have a large site with more than 50,000 URLs, you should create multiple Sitemap's and submit a sitemap index file.
So we changed it as,
renamed sitemap.xml, sitemap1.xml and sitemap1.xml, sitemap2.xml
created sitemap index page with sitemap filename(added sitemap1 & sitemap2 in this page)
<?xml version="1.0" encoding="UTF-8"?>
As to surprise Google Webmaster tools is showing Errors in sitemap1, sitemap2.
So we did replace as it was before, renamed sitemap1, sitemap2 to sitemap, sitemap1(Previous pages).
Worst thing it now still is showing "Error"
Please help me, What i should do....