This will have disappeared through etiher:
css: the nav is there, just hidden
code: the code has been deleted or commented out
https://developer.wordpress.org/themes/functionality/pagination/
Identifying what and how is impossible for anyone to debug without access to code and dev environment.
@whitsey We have managed to resolve the issue without editing CSS or debugging code. Pleased to say it was a Setting that simply needed changing. Go into your site’s admin Dashboard and select Manage → Settings → Writing (this should take you to https://wordpress.com/settings/writing/yourwebsite.com ), where you can change the setting for “Infinite Scroll” to “Load more posts using the default theme behaviour“.