[Plugin: Google XML Sitemaps] Error writing my sitemap file

Hi,
I need help please.
I get the following errors:
“There was a problem writing your sitemap file. Make sure the file exists and is writable. Learn more
There was a problem writing your zipped sitemap file. Make sure the file exists and is writable. Learn more”
I have checked, a file exist.
If I try to rebuild the sitemap manually I get the same error.
I have even put both files, sitemap.xml and sitemap.xml.gz on 777 and still nothing. However I have tried to open the sitemap.xml and I get the following message:
“XML document must have a top level element. Error processing resource ‘http://www.pixazy.com/sitemap.xml’.”

It used to work before.
Please advise what to do.
Thank you,
Cornel Achi

http://wordpress.org/extend/plugins/google-sitemap-generator/