Error Messages?

What could cause these error messages?

[22-May-2017 03:01:12 UTC] PHP Warning:  DOMDocument::load(): I/O warning : failed to load external entity "/home/website/public_html/examplecom/wp-content/plugins/google-sitemap-generator/sitemap.xsl" in /home/website/public_html/examplecom/wp-content/plugins/google-sitemap-generator/sitemap-core.php on line 1658
[22-May-2017 03:01:12 UTC] PHP Warning:  XSLTProcessor::importStylesheet(): compilation error in /home/website/public_html/examplecom/wp-content/plugins/google-sitemap-generator/sitemap-core.php on line 1662
[22-May-2017 03:01:12 UTC] PHP Warning:  XSLTProcessor::importStylesheet(): xsltParseStylesheetProcess : empty stylesheet in /home/website/public_html/examplecom/wp-content/plugins/google-sitemap-generator/sitemap-core.php on line 1662