Multisite: Main Site is down with Error 500

Hello, I have the multisite with main site 1attimo.net (now is down) and second site appuntamenti.1attimo.net is Up.

I don't know when main site go down, but ... I install before one plugin free for 1 week (after need to pay) , and after I decided I did not want it anymore.
I did not see my site for a few days.
Now it is down.
But only main site. The second site is Up, it is only test.

What can I do, guys?
Thank you
Olga