Plug-in fail – Help me

Hello.

Today i installed a plug-in and activated it. after that i coudn’t open my wordpress site again.

The site still works fine, i just can’t log in and change anything, which i what i want to.

I read on this forum that i “just” had to deactivate it and it would be fine. But the problem is that i can’t log in again and deactivate it.

What do i do?