Web application throwing error(ERR_EMPTY_RESPONSE) codeigniter website

I can see lots of suggestions to resolve this but as am new to this would like to get a easy straight forward view to solve this issue.

MY URL contains https before the domain name. Also some of the pages were working in the site where only few where I used to get data from API is throwing this error

The xxxxxx page isn’t working

xxxxxx didn’t send any data.

ERR_EMPTY_RESPONSE

many thanks.