Solution:
If you have entire backup of your previous WordPress site and database. upload new WordPress and replace wp-content of its with new one.
try checking it if you still that error come. rename themes and plugin directory inside wp-content. and also rename you current theme directory i.e Avada
after that admin panel will be accessible to you. now replace your theme and plugin directory name to actual and reset permalink from Adminpanel=>settings=>permalink=>pretty permalink=