Login not working?

A friend asked me to update his site, but has no real information on Drupal or editing the site. Someone else built the site for him. He could not tell me how/where to login to the backend of the site. The site has not been updated in years, and is very broken. I am supposed to login and figure out whether it’ll be easy to fix or not. (I assume that a couple years of not running updates is the biggest issue…)

I did some searching (not gonna lie- I haven’t worked in Drupal yet, but have been working with WordPress sites for years) and found that most people recommend trying http://www.yoursitename.com/?q=user/login. That worked to take me to a login page, but regardless of what I enter (he gave me numerous usernames and passwords that it “might be”) it keeps giving me an error:
“The website encountered an unexpected error. Please try again later.”

1) I can’t tell if the login that I am seeing is the login for the Drupal backend of the site or the login for the forum that he had on the site (which is currently broken)?
2) I can’t tell if he just gave me the wrong username/password?
3) Is there a different way to determine the login page for the Drupal backend?
4) Is it possible that the website is so broken that I can’t even login at all?

Thanks for any insight.

Source: https://www.drupal.org/taxonomy/term/21/feed