Update:The issue is resolved. Followed the patch before but didn't get working so posted here. I tried it again today and it worked.
From the last two days my XML sitemap is not being updated after the cron runs. I used to have 2 pages for the XML Sitemap, and Google used to crawl the website for the two pages. But, Now I am able to see only one page of the sitemap and the second page is missing.I tried to run cron manually, but it is of no use.
My website is Click Here
When I try to Update the Cached data, below is the error message
An AJAX HTTP error occurred. HTTP Result Code: 500 Debugging information follows. Path: /batch?render=overlay&id=3670&op=do StatusText: Service unavailable (with message) ResponseText: XMLSitemapGenerationException: Unknown error occurred while writing to file public://xmlsitemap/NXhscRe0440PFpI5dSznEVgmauL25KojD7u4e9aZwOM/1.xml. in XMLSitemapWriter->endDocument() (line 157 of /home/{username}/public_html/sites/all/modules/xmlsitemap/xmlsitemap.xmlsitemap.inc).
This is the error in Webmaster tools of Google
General HTTP error: 404 not found
Sitemap: www.telugism.com/sitemap.xml?page=2
HTTP Error: 404