There could be any number of problems with your site, so it’s really hard to say what the real “problem” is.
I’d suggest first checking out your console for http request errors (see if there are any files not transferring), and also try logging in to see if anything changes.
Based on what you’re describing, though, it sounds like you’re experiencing the same problems as this other user:
https://stackoverflow.com/questions/18769141/wordpress-admin-not-loading-css-js
Perhaps give that noted fix a shot, and see if that works.