@simonagradinaru Welcome to the Netlify community.
Let’s start here:
If the CSS file is where it’s supposed to be and your pages have the correct links to that file, but the file is not being delivered properly, that is indeed a problem.
If, on the other hand, the CSS file is missing and/or the links to the CSS file are not correct, that’s something you’ll have to correct.
However, without more information it’s very difficult for others to help you.