Can I access the code with my admin I am an in-house graphic designer with virtually no experience with building websites.
My company has a website built with Craft for which we have an admin to change some content like images and tables. 
Some pages are not customizable at all. It seems they might have been constructed only with CSS and HTML. I don't know.
Now, we need to change these pages and build new ones using the same structure.
The agency that built the website seems to be ghosting on us so we have to find a solution to make the changes ourselves.

How do I access the code with my admin? 
Is is possible that the guy who built the site restricted our access to the code?
The template and CSS code cannot be accessed from the admin. You'll need to gain access to the git repository that (hopefully) exists with your site's code. 

Or in a less-preferable scenario, you could access the server via FTP and modify the templates and CSS there.

You might want to take a read through the Craft CMS docs as a starting point to get a little better idea of how the files are structured.