getting error message from google with wordpress sites

I got the message below from google today on quite a few of my wordpress sites. nothing has been changed on them. can you please tell me what is causing this?

Googlebot cannot access CSS and JS files on

To: Webmaster of,

Google systems have recently detected an issue with your homepage that affects how well our algorithms render and index your content. Specifically, Googlebot cannot access your JavaScript and/or CSS files because of restrictions in your robots.txt file. These files help Google understand that your website works properly so blocking access to these assets can result in suboptimal rankings.