My website has been hacked

I am looking after at a very old WordPress website that has not been updated in long time, But I can’t update the website now, when I go to install new image this is the Warning: in_array() expects parameter 2 to be array, null given in /home/michael1/public_html/wp-includes/query.php on line 2399 and when I try to update the WordPress software the website is gone. The Following error message comes up;
Fatal error: Call to undefined method Arras_Widget_Tag_Cloud::WP_Widget_Tag_Cloud() in /home/michael1/public_html/wp-content/themes/arras/library/widgets.php on line 404
Can to tell me how can I fix this major problem?
Please help me!
Sheena