This error generally caused by the Theme when you update the wordpress version to 4.2 because the theme is not yet compatible with version.
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 .