Hello and welcome @Sergej_Županić 
Could you try using safe mode to access your site? [YourSite]/safe-mode That may help you track down what’s going wrong with it.
You may also be able to see some errors in your browser console when you try and access your site too. Your /logs page may also have some information.
Hopefully that will help you narrow down what the issue is, or at least give some extra information you can share here. 