Error establishing a database connection when i try to log in to the dashboard.

Try login in: https://www.addisphotofix.com/donation/wp-login.php

It returns the same error and wp-admin link
https://www.addisphotofix.com/donation/wp-admin/
after that when i go to https://www.addisphotofix.com/donation/
it shows me i am logged in.
What could be the possible reasons?

Can you FTP into your site or login and revert to Twenty Eleven theme by renaming your current theme. Perhaps that will solve the problem.

uploading wp-admin folder and removing the previous one solves the problem, thanks.

Glad to see that you could solve the problem.

Thank you mate.