how to fix robots.txt problem in webmaster tools and analytics?

Hello,

Last week I switched my theme from atahualpa to catalyst. Since then I received a message from webmaster tools telling me that googlebot cant access my site nor my robots.txt file.

By the way, my traffic also felt to zero in google analytics.

I really don't have knowledge about this topic, please give me advise on what should I do to get everything back to normality.

I also added yoast seo plugin which auto generates a xml sitemap but I don't know what to do with it :slight_frown:

Thank you for your help,
Luciano