I just tried to submit sitemap.xml to Google Webmaster Tools.

But it gives following error
URLs restricted by robots.txt

We encountered an error while trying to access your Sitemap. Make sure that your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit.
But there is no robots.txt file in my directory.Even I tried to add a new robot. txt file without restrictions.
No success.

Create a new robots.txt file with the following content:

User-agent: *
Disallow:

Sitemap: {Your sitemap URL}

Wait for Google to re-download the robots.txt file (usually every 24 hours), and see if that sorts the problem.