What to do, when suddenly Search page is not working or clicking on the settings panel getting blank page with error?

if you know the below information then you can forward this information to support 

1. since when you are getting that error?

2. what changes you did that lead to that error?

3. did you delete or removed anything from the reactive builder? if yes what it is exactly.

otherwise,  if you don't know the above information then you will have to recreate the shortocde i think as it will be the best solution and less time consuming.

The most common scenario is you have deleted some data from your wordpress admin but you have selected that that data in your reactive search page settings pannel. as a result when you are going to open the settings pannel its causing the error. so your best shot at this moment is to revert the deleted changes or recreate the shortcode and build the search page again. 

NB: Please note that, in most of the cases support won't be able to help you for such errors.