Mario on "Can't access wp-admin"

I there,
I was trying a theme on my personal website, which I often use to test out stuff. Anyway, I installed a new theme and the site gave me an error page.
Via FTP I removed the theme so that wP could go back to the previous, which it did. The site is in effect restored and working, however I cannot access the dashboard. When I try to go to wp-admin it gives me an error page.
What shall I do?
Any suggestion?