I can't access my page as administrator.

Hi!
I can’t access my page as administrator.

I have downloaded an Advertise plugin and since that I can’t access my site any more.

If you go to my page here: https://www.i-dont-know-how-to.com you will see a message above saying “warning”.
But the website still works.
When I tried to access https://www.i-dont-know-how-to.com/wp-admin show up a blank page with the same message.

Don’t know what to do. Does exist Virus Plugins?

I am completly new here. Please someone help me.