Here is two solution to resolve it :
1 ) You can manually disable the theme on your website by using FTP client or login to your cPanel ==> file Manager . You will just need to rename the theme . Once you have rename it your website will work fine and then login to your Wp-admin section and check if there is any update available for the theme, If yes then update it which should resolve your issue .
2) This is a temporarily fix for this issue. Check the error log which will tell you which line contains the error and which file , comment out the entire section (function) that is causing the error by placing /* in front of the statement and */ immediately following it. This will exclude it from being processed.
That’s it .